Allow non-superuser to cancel superuser tasks.

Hi hackers!

In our Cloud we have a patch, which allows non-superuser role ('mdb_admin')
to do some superuser things.
In particular, we have a patch that allows mdb admin to cancel the
autovacuum process and some other processes (processes with
application_name = 'MDB'), see the attachment.
This is needed to allow non-superuser roles to run pg_repack and to cancel
pg_repack.
We need to cancel running autovac to run pg_repack (because of locks), and
we need to cancel pg_repack sometimes also.

I want to reduce our internal patch size and transfer this logic to
extension or to core.
I have found similar threads [1] and [2], but, as far as I understand, they
do not solve this particular case.
I see 2 possible ways to implement this. The first one is to have hool in
pg_signal_backend, and define a hook in extension which can do the thing.
The second one is to have a predefined role. Something like a
`pg_signal_autovacuum` role which can signal running autovac to cancel. But
I don't see how we can handle specific `application_name` with this
solution.

[1]
https://www.postgresql.org/message-id/flat/F9408A5A-B20B-42D2-9E7F-49CD3D1547BC%40enterprisedb.com
[2]
https://www.postgresql.org/message-id/flat/20220722203735.GB3996698%40nathanxps13