Add new protocol message to change GUCs for usage with future protocol-only GUCs

Currently the only way to set GUCs from a client is by using SET
commands or set them in the StartupMessage. I think it would be very
useful to be able to change settings using a message at the protocol
level. For the following reasons:

1. Protocol messages are much easier to inspect for connection poolers
than queries
2. It paves the way for GUCs that can only be set using a protocol
message (and not using SET).
3. Being able to change GUCs while in an aborted transaction.
4. Have an easy way to use the value contained in a ParameterStatus
message as the value for a GUC

I attached a patch that adds a new protocol message to set a GUC
value. There's definitely still some more work that needs to be done
(docs for new libpq APIs, protocol version bump, working protocol
version negotiation). But the core functionality is working. At this
point I'd mainly like some feedback on the general idea.

The sections below go into more detail on each of the reasons I mentioned above:

(1) PgBouncer does not inspect query strings, to avoid having to
write/maintain a SQL parser (even a partial one). But that means that
clients cannot configure any behaviour of PgBouncer for their session.
A few examples of useful things to configure would be:
a. allow changing between session and transaction pooling on the same
connection.
b. intercepting changes to search_path, and routing different schemas
to different machines (useful for Citus its schema based sharding).
c. intercepting changing of pgbouncer.sharding_key, and route to
different machines based on this value.

(2) There are currently multiple threads ongoing that propose very
similar protocol changes for very different purposes. Essentially all
of them boil down to sending a protocol message to the server to
change some other protocol behaviour. And the reason why they cannot
use GUCs, is because the driver and/or connection pooler need to know
what the setting is and be able to choose it without a user running
some SQL suddenly changing the value. The threads I'm talking about
are: Choosing specific types that use binary format for encoding [1].
Changing what GUCs are reported to the client using ParameterStatus
(a.k.a configurable GUC_REPORT) [2]. Changing the compression method
that is used to compress messages[3].

Another benefit could be to allow a connection pooler to configure
certain settings to not be changeable with SQL. For instance if a
pooler could ensure that a client couldn't later change
session_authorization, it could use session_authorization to set the
user and then multiplex client connections from different users over
the same connection to the database.

(3) For psql it's useful to be able to control what messages it gets a
ParameterStatus for, even when the transaction is in aborted state.
Because that way it could decide what parameters status updates to
request based on the prompt it needs to display. And the prompt can be
changed even during an aborted transaction.

(4) PgBouncer uses the value contained in the ParameterStatus message
to correctly set some GUCs back to their expected value. But to do
this you currently need to use a SET query, which in turn requires
quoting the value using SQL quoting . This wouldn't be so bad, except
that GUC_LIST_QUOTE exists. Parameters with GUC_LIST_QUOTE have each
item in the list returned **double** quoted, and then those double
quoted items separated by commas. But to be able to correctly set
them, they need to be given each separately **single** quoted and
separated by commas. Doing that would require a lot of parsing logic
to replace double quotes with single quotes correctly. For now
pgbouncer only handles the empty string case correctly, for the
situations where the difference between double and single quotes
matters[4].

[1]: https://www.postgresql.org/message-id/flat/CA%2BTgmoZyAh%2BhdN8zvHeN40n9vTstw8K1KjuWdgDuAMMbFAZqHg%40mail.gmail.com#e3a603bbc091e796148a2d660a4a1c1f
[2]: https://www.postgresql.org/message-id/flat/CAFj8pRBFU-WzzQhNrwRHn67N0Ug8a9-0-9BOo69PPtcHiBDQMA(at)mail(dot)gmail(dot)com
[3]: https://www.postgresql.org/message-id/flat/AB607155-8FED-4C8C-B702-205B33884CBB%40yandex-team.ru#961c695d190cdccb3975a157b22ce9d8
[4]: https://github.com/pgbouncer/pgbouncer/blob/fb468025d61e1ffdc6dbc819558f45414e0a176e/src/varcache.c#L172-L183

P.S. I included authors and some reviewers of the threads I mentioned
for 2 in the CC. Since this patch is meant to be a generic protocol
change that could be used by all of them.