[codegen] Add fluss-codegen module for runtime code generation

[platinumhamburg]

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Motivation

Runtime code generation is essential for achieving high performance in data processing systems. Instead of using reflection or generic implementations, generated code can:

1. Eliminate virtual method dispatch - Direct field access and type-specific operations
2. Enable JIT optimization - Generated code can be better optimized by the JVM
3. Reduce boxing/unboxing overhead - Type-specific code avoids primitive boxing
4. Support complex type comparisons - Nested types (arrays, maps, rows) require specialized comparison logic

The initial use case is RecordEqualiser for comparing InternalRow instances, which is needed for:

• Change data capture (CDC) deduplication
• Aggregation state management
• Primary key table updates

Solution

Core Framework

Component	Description
CodeGeneratorContext	Manages reusable code fragments, member variables, and class-level declarations
JavaCodeBuilder	Type-safe builder for constructing Java source code with fluent API
CompileUtils	Compiles generated source code using Janino with LRU caching
GeneratedClass<T>	Wrapper holding generated source code and compiled class
CodeGenException	Exception type for code generation failures

Type-Safe API

• Modifier enum: PUBLIC, PRIVATE, PROTECTED, STATIC, FINAL, ABSTRACT, SYNCHRONIZED, VOLATILE, TRANSIENT
• PrimitiveType enum: BOOLEAN, BYTE, CHAR, SHORT, INT, LONG, FLOAT, DOUBLE, VOID
• Param class: Type-safe method parameter representation
• Helper methods: mods(), params(), typeOf(), arrayOf()

Code Generators

Generator	Output Interface	Description
EqualiserCodeGenerator	RecordEqualiser	Generates code for comparing two InternalRow instances

Supported Data Types

The EqualiserCodeGenerator supports all Fluss data types:

• Primitive types: BOOLEAN, TINYINT, SMALLINT, INT, BIGINT, FLOAT, DOUBLE
• String types: CHAR, VARCHAR, STRING
• Binary types: BINARY, VARBINARY, BYTES
• Temporal types: DATE, TIME, TIMESTAMP, TIMESTAMP_LTZ
• Numeric types: DECIMAL (with precision/scale)
• Complex types: ARRAY, MAP, ROW (nested)

Features

• Field projection support for partial row comparison
• Compiled class caching with configurable cache size
• Janino dependency shaded to org.apache.fluss.shaded.org.codehaus.janino to avoid classpath conflicts
• Comprehensive Javadoc and package-info documentation

Anything else?

No response

Willingness to contribute

• I'm willing to submit a PR!