feat(rpc): add stream and keepalive limits to indexer gRPC server

[oxarbitrage]

Summary

• Closes Indexer gRPC server uses default tonic settings with no stream/connection limits #10734
• Add max_concurrent_streams, http2_keepalive_interval, and http2_keepalive_timeout to the indexer gRPC server
• Values are hardcoded constants — the primary clients (Zaino and Zallet) each need 1-2 streams, no use case for user configuration
• Intentionally omits max_connection_age — both clients use long-lived syncer connections that would break if the server forced disconnection
• Omits tcp_keepalive — tonic ignores it when using serve_with_incoming, and HTTP/2 keepalive already detects dead connections at the application layer
• Note: feat(rpc): add lightwalletd-compatible gRPC server implementing CompactTxStreamer #10953 adds a second tonic server that would benefit from the same limits