feat: add query-gateway plugin for safe caching of read-only body queries

[fatihbm]

Description

Adds the query-gateway plugin, an opt-in cache boundary for read-only HTTP requests that carry query content in the request body.

The plugin supports both client POST and RFC 10008 QUERY requests:

• Client POST requests are preserved when sent upstream.
• Client QUERY requests are forwarded as POST by default for upstream compatibility.
• Routes with upstream QUERY support can preserve the QUERY method.
• Equivalent POST and QUERY query payloads produce the same canonical cache key.
• Cache admission is conservative and explicit; arbitrary POST traffic is never cached.
• Local-memory caching is supported by default, with optional Redis and Redis Cluster backends.
• Redis failures use a bounded local-memory fallback.
• Cache safety rules exclude unsafe responses and requests with relevant cache-control, authorization, cookie, or set-cookie conditions.

This is useful for search, analytics, reporting, Elasticsearch, and ClickHouse-style read-only requests. These workloads often use POST because they require a request body, even when their semantics are safe and idempotent. The plugin enables safe caching today without requiring immediate native QUERY support from every upstream.

Which issue(s) this PR fixes:

Fixes #13807

Validation

• Added unit and integration coverage for request method handling, canonical cache keys, cache admission, cache hit/miss behavior, bypass behavior, and fallback behavior.
• Added an Elasticsearch integration profile that verifies correlated client, APISIX, and upstream logs.
• Verified client POST and QUERY scenarios, upstream POST and native QUERY forwarding modes, and cache bypass cases.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible