Support pg_logical_emit_message

This function is used to write arbitrary messages into the WAL that can then be read by logical decoding plugins. This is very useful to transactionally produce messages to Kafka. It's also a really great way to heartbeat and advance replication slots for databases with low write volume. This is currently supported by wal2json, but isn't supported by pgoutput yet. Here are some example payloads from wal2json: https://github.com/eulerto/wal2json/blob/master/expected/message.out