reverse_tunnels: add gRPC streaming client for reverse tunnel reporting

[aakugan]

BUILDS on #44342. Please review that one first.
INCREMENTAL DIFFS

Commit Message

Add gRPC streaming client for reverse tunnel reporting

Additional Description

Adds a bidirectional gRPC streaming client that pushes reverse tunnel connection state to a remote ReverseTunnelReportingService. This is a client implementation for the EventReporter introduced in the previous PR.

Protocol:

• On connect (and every reconnect) the client does a full state push of all known connections obtained from the reporter.
• Between connects the client sends incremental diffs on a periodic send timer. Empty messages serve as heartbeats.
• Each request carries an incrementing nonce. The server ACKs by echoing the nonce; a NACK carries an error_detail and triggers disconnect.
• If too many nonces remain unacked the client disconnects and reconnects, treating the server as dead or stuck.
• The server may adjust the send interval via report_interval in its ACK; the client floors it at 25ms to prevent tight loops.

Key components:

• GrpcClient: implements ReverseTunnelReporterClient and Grpc::AsyncStreamCallbacks. Manages stream lifecycle, nonce tracking, send/retry timers, and per-cluster stats.
• GrpcClientFactory: dynamically registered factory resolved by the EventReporter when constructing clients from config.
• GrpcClientConfig: parsed config struct with defaults (5s send interval, 5s retry, 5 max retries, 1M buffer).

Additional considerations:

• initialized_ guard prevents silent event queuing if cluster lookup or client creation fails during onServerInitialized().
• Buffer overflow while disconnected does not re-arm the retry timer.
• Nonce reset on every new stream to avoid stale ACK confusion.

Risk Level: Low

Testing

Unit tests

[repokitteh-read-only]

CC @envoyproxy/dependency-shepherds: Your approval is needed for changes made to (bazel/.*repos.*\.bzl)|(bazel/dependency_imports\.bzl)|(api/bazel/.*\.bzl)|(.*/requirements\.txt)|(.*\.patch).
envoyproxy/dependency-shepherds assignee is @phlax

🐱

Caused by: #44344 was opened by aakugan.

see: more, trace.