feature(server): add ConnectionPoolController

[Oscarcheng0312]

• [x] I have read the CONTRIBUTING.md guidelines.
• [x] I have registered the PR changes.

Ⅰ. Describe what this PR did

Implemented private-protocol-based connection pool metrics reporting and server-side REST APIs.

• Client (RM) side:

  • Enhanced RmNettyRemotingClient to include ConnectionPoolInfo in heartbeat messages (HeartbeatMessage.PING).
  • Reports pool metrics periodically (default: 30s) through a private channel without exposing public HTTP endpoints.

• Server (TC) side:

  • ServerHeartbeatProcessor parses and caches ConnectionPoolInfo from incoming heartbeat messages.
  • Added access methods in NettyRemotingServer to fetch all or per-client pool info.

• REST endpoints:

  • Added ConnectionPoolController under /api/v1/connection-pool:
    • GET /info — Retrieve connection pool info of all clients.
    • GET /info/{clientAddress} — Retrieve connection pool info of a specific client.
    • GET /all — Retrieve locally collected data source pool metrics via DataSourceConnectionPoolCollector.

• Server integration:

  • Registered NettyRemotingServer in Spring context and injected it into ConnectionPoolController for REST access.

Backward-compatible: older clients still send standard heartbeat messages, and the server responds normally.

Ⅱ. Does this pull request fix one issue?

Yes, it fixs #7575 .

Ⅲ. Why don't you add test cases (unit test/integration test)?

• The changes involve both network-level heartbeat communication and in-memory caching logic, which are more suitable for integration testing.
• Test cases will be added.

Ⅳ. Describe how to verify it

1. Start the Seata server.
2. Run an RM client using this version of RmNettyRemotingClient.
3. After ~30 seconds:
   • Access GET /api/v1/connection-pool/info to view all active client pool info.
   • Access GET /api/v1/connection-pool/info/{clientAddress} to query a specific client (use _ instead of : in path).
   • Access GET /api/v1/connection-pool/all to verify locally collected pool metrics.
4. Verify that:
   • New clients report valid metrics.
   • Old clients work as before (no metrics available but heartbeat succeeds).

Ⅴ. Special notes for reviews