Add 'worker_type' to pg_stat_subscription

Hi hackers,

Earlier this year I proposed a small change for the pg_stat_subscription view:

------
...it would be very useful to have an additional "kind" attribute for
this view. This will save the user from needing to do mental
gymnastics every time just to recognise what kind of process they are
looking at.
------

At that time Amit replied [1] that this could be posted as a separate
enhancement thread.

Now that the LogicalRepWorkerType has been recently pushed [2]
(something with changes in the same area of the code) it seemed the
right time to resurrect my pg_stat_subscription proposal.

PSA patch v1.

Thoughts?

------
[1] https://www.postgresql.org/message-id/CAA4eK1JO54%3D3s0KM9iZGSrQmmfzk9PEOKkW8TXjo2OKaKrSGCA%40mail.gmail.com
[2] https://github.com/postgres/postgres/commit/2a8b40e3681921943a2989fd4ec6cdbf8766566c

Kind Regards,
Peter Smith.
Fujitsu Australia