feat: add an explain method to show generated plans

[ChunxuTang]

This PR

• adds an explain_datafusion method to show the generated plans (graph logical plan, DataFusion logical plan, DataFusion physical plan) in the DataFusion-based query execution pipeline. It is similar to the EXPLAIN method in DataFusion.
• adds a test file with examples to show the output. To explicitly see the plan results, run cargo test --test test_explain_output -- --nocapture --test-threads=1

Example query and output:

MATCH (p:Person) RETURN p.name, p.age

+--------------------+----------------------------------------------------------------------------------------------------------------------+
| plan_type          | plan                                                                                                                 |
+--------------------+----------------------------------------------------------------------------------------------------------------------+
| graph_logical_plan | Project {                                                                                                            |
|                    |     input: ScanByLabel {                                                                                             |
|                    |         variable: "p",                                                                                               |
|                    |         label: "Person",                                                                                             |
|                    |         properties: {},                                                                                              |
|                    |     },                                                                                                               |
|                    |     projections: [                                                                                                   |
|                    |         ProjectionItem {                                                                                             |
|                    |             expression: Property(                                                                                    |
|                    |                 PropertyRef {                                                                                        |
|                    |                     variable: "p",                                                                                   |
|                    |                     property: "name",                                                                                |
|                    |                 },                                                                                                   |
|                    |             ),                                                                                                       |
|                    |             alias: None,                                                                                             |
|                    |         },                                                                                                           |
|                    |         ProjectionItem {                                                                                             |
|                    |             expression: Property(                                                                                    |
|                    |                 PropertyRef {                                                                                        |
|                    |                     variable: "p",                                                                                   |
|                    |                     property: "age",                                                                                 |
|                    |                 },                                                                                                   |
|                    |             ),                                                                                                       |
|                    |             alias: None,                                                                                             |
|                    |         },                                                                                                           |
|                    |     ],                                                                                                               |
|                    | }                                                                                                                    |
| logical_plan       | Projection: p__name AS p.name, p__age AS p.age                                                                       |
|                    |   Projection: person.person_id AS p__person_id, person.name AS p__name, person.age AS p__age, person.city AS p__city |
|                    |     TableScan: person                                                                                                |
| physical_plan      | ProjectionExec: expr=[name@0 as p.name, age@1 as p.age]                                                              |
|                    |   DataSourceExec: partitions=1, partition_sizes=[1]                                                                  |
+--------------------+----------------------------------------------------------------------------------------------------------------------+

[codecov-commenter]

Codecov Report

❌ Patch coverage is 87.07865% with 23 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@60e7756). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
rust/lance-graph/src/query.rs	87.07%	23 Missing ⚠️

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