[RIP-2] Add proxy admin online client query API

[pilichoumao]

Summary

This draft proposes the formal protobuf contract for RocketMQ RIP-2 Proxy Admin
M1 online-client query. It adds a standalone ProxyAdminService to
apache/rocketmq/v2/admin.proto with ListClients, DescribeClient,
ListClientsByGroup, and ListClientsByTopic.

The service is separate from MessagingService so the implementation can
isolate admin service registration, port, executors, authentication, and ACL.

Review Checkpoints

• API proposal head: c372905ce927cf8957333e7ac07877f295fd7ec9
• Downstream RocketMQ head: bde34c88ba575177b28e2854a8e7fa20ee628a82
• Downstream implementation PR: [RIP-2] Add proxy admin online client query endpoint rocketmq#10603
• RIP-2 issue: [RIP-2] Proxy Admin gRPC Interface Surface (M1: Online Client Query Module) rocketmq#10599

M1 Scope

• PROXY_SCOPE_LOCAL_PROXY and omitted scope are the supported public M1 modes.
• PROXY_SCOPE_ALL_PROXIES and PROXY_SCOPE_PROXY_ID are reserved but remain
  gated until multi-proxy discovery, authorization, timeout, and pagination
  ownership are accepted.
• The downstream ACL resource is Admin:proxy.admin.client.

Downstream Verification

Focused: Tests run: 57, Failures: 0, Errors: 0, Skipped: 0
Broad: Tests run: 779, Failures: 0, Errors: 0, Skipped: 0
Package: BUILD SUCCESS

The downstream full release-distribution smoke also reached all four generated
public RPCs on the isolated admin listener, verified live NOT_FOUND and
BAD_REQUEST semantics, and proved service isolation in both directions.
Its authenticated runtime runner additionally exited 0 after checking valid,
unsigned, and bad-signature calls plus LIST/GET ACL separation on
Admin:proxy.admin.client.

Core JaCoCo evidence:

• service.admin.client instruction 92.93%, branch 86.62%, line 94.41%
• grpc.v2.admin instruction 92.81%, branch 85.79%, line 94.73%

The 1M-client generated public gRPC deep full-range pagination run measured
P99 0.843 ms under a fixed 4 GiB heap, with zero swaps and no OOM.

Submission guard:

python3 dev/rip2_submission_guard.py --check-remote --check-apis-remote --check-github
RIP-2 submission guard passed.

External Gates

• The downstream branch currently uses
  org.apache.rocketmq:rocketmq-proto:2.2.0-rip2-SNAPSHOT.
• Community acceptance and publication as an official artifact are required
  before the implementation can replace the local contest artifact.
• Dashboard CLIENT-01 joint E2E remains external to these repositories.
• The fork PR workflows currently conclude action_required; Apache
  maintainer approval is required before hosted checks can start.