Add a hook for handling logical decoding messages on subscribers.

Hi all,

Commit ac4645c015 allows pgoutput to send logical decoding messages,
but it's limited to applications that use the pgoutput plugin -- the
built-in logical replication doesn't use it. I'd like to propose
introducing a hook to the logical replication message handling so that
extensions can plug in their own handling routine. This feature can be
used for extensions to implement DDL replication, function
replication, or trigger user-specific routines on the subscriber side.

I've attached the PoC patch; it adds a hook function, and adds a new
'message' subscription option that allows the user to request the
publisher to send logical decoding messages. Therefore, users need to
enable the 'message' option and set up the hook function at server
startup in order to receive the messages and trigger the hook
function.

I I went with a hook function in the patch. While it lets you chain
the multiple hook functions, providing the registration API might be
better, or other types of registry can also be considered.

Feedback is very welcome.

Regards,

--
Masahiko Sawada
Amazon Web Services: https://aws.amazon.com