[kafka] Add request dispatch and transport framework

[gyang94]

Purpose

Closes #4264.

Part of umbrella #4185; this PR does not close the umbrella.

This is the first foundation stage under #4185: request/transport framework, followed by ApiVersions and Metadata. Each stage is tracked by a separate child issue. This PR excludes CreateTopics/DeleteTopics.

Brief change log

• Introduce request context, typed API specifications and handlers, a frozen capability registry, asynchronous dispatch, and common error mapping.
• Propagate the listener context through the Kafka channel and recognize configured protocol endpoints in NettyServer.
• Make request buffer release idempotent and release the response buffer when serialization fails.
• Add isolated dispatcher, registry, decoder, and buffer-ownership regression tests.
• Preserve main's existing request-handler implementation in this stage. The ApiVersions follow-up wires the new dispatcher into production.

Scope: 16 files, +1039 / -41 lines including tests. This PR adds no concrete API handler, Produce implementation, topic administration, dependency, or storage format.

Tests

• mvn -o -pl fluss-rpc,fluss-kafka verify
• Kafka: 17 unit tests and 1 integration test.
• RPC: full module unit/integration suites, including NettyServer and authentication coverage; 2 existing tests are skipped.
• git diff --check; git range-diff confirms unchanged patches after rebasing onto main (b9a1653fa).

Dependencies were rebuilt with mvn -o -pl fluss-kafka -am install -DskipTests on Java 11. This also passed the reactor's Checkstyle, Spotless and RAT checks. The entire repository test suite was not run.

API and Format

Adds internal Kafka extension points; existing Fluss public APIs and storage formats are unchanged. Concrete ApiVersions and Metadata behavior is introduced only in the two follow-ups. Tables remain managed by Fluss DDL.

Documentation

This PR description documents the stage boundary and review order. No end-user Produce or schema-format documentation is introduced here.

Stack

1. Request/transport framework — this PR.
2. ApiVersions — [kafka] Serve ApiVersions from registered capabilities #4260.
3. Metadata — [kafka] Implement Metadata API over Fluss table metadata #4261.

Generative AI disclosure: Codex assisted with implementation, testing, and commit organization. Human review is required before merge.