Add grpc keepalive configuration. (#19339)
Prior to the introduction of this configuration, grpc keepalive messages were
sent after 2 hours of inactivity on the stream. This posed issues in various
scenarios where the server-side xds connection balancing was unaware that envoy
instances were uncleanly killed / force-closed, since the connections would
only be cleaned up after ~5 minutes of TCP timeouts occurred. Setting this
config to a 30 second interval with a 20 second timeout ensures that at most,
it should take up to 50 seconds for a dead xds connection to be closed.
