Vector Sink Http2 Keepalive.Enhancement

The vector sink now supports optional HTTP/2 keepalive on its pooled gRPC connections, configured via a new keepalive block (interval_secs and timeout_secs). When enabled, the sink periodically sends HTTP/2 PING frames so that a connection to a downstream Vector instance that has gone away is detected and evicted rather than reused indefinitely (which could otherwise stall delivery until the connection was replaced). Keepalive is disabled by default; when enabled, interval_secs defaults to 60 (aligned with gRPC keepalive guidance to avoid tripping too_many_pings policies) and timeout_secs defaults to 20. PINGs are sent on idle connections so an idle-but-dead connection is still detected.

authors: graphcareful