Draft: asynchronous phase handler

[ensh63]

Draft implementations of the following items are added to SDK:

• asynchronous phase handler;
• asynchronous subrequest;
• type-safe request context;
• typed storage for Pool.

[Copilot]

Pull request overview

This PR introduces asynchronous capabilities to the Rust SDK for nginx modules, including async phase handlers, subrequests, and type-safe request context management.

Key Changes:

• Introduces AsyncHandler trait and supporting infrastructure for asynchronous HTTP request handling
• Adds RequestContext trait for type-safe request-specific context data management
• Implements TypeStorage trait for storing typed values in nginx memory pools
• Refactors existing examples to use new HttpRequestHandler trait and add_phase_handler function

Reviewed changes

Copilot reviewed 19 out of 20 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/log.rs	Extracts connection to intermediate variable in logging macro
src/http/status.rs	Adds From<HTTPStatus> implementation for ngx_int_t
src/http/request_context.rs	Implements RequestContext trait for managing request-specific context
src/http/request.rs	Adds HttpHandlerReturn and HttpRequestHandler traits, new request methods
src/http/mod.rs	Adds async_request module exports (feature-gated)
src/http/conf.rs	Adds HttpPhase enum and add_phase_handler function
src/http/async_request.rs	Implements async handler infrastructure and subrequest builder
src/core/type_storage.rs	Implements TypeStorage trait for pool-based type storage
src/core/pool.rs	Adds remove method for cleaning up pool-allocated values
src/core/mod.rs	Exports new type_storage module
examples/*.rs	Refactors examples to use new handler registration pattern
examples/config	Adds async_request module configuration
examples/Cargo.toml	Adds async-related dependencies
Cargo.toml	Adds futures dependency
.github/workflows/nginx.yaml	Updates CI to load async_request module

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