Make wal_receiver_timeout configurable per subscription

Hi,

When multiple subscribers connect to different publisher servers,
it can be useful to set different wal_receiver_timeout values for
each connection to better detect failures. However, this isn't
currently possible, which limits flexibility in managing subscriptions.

To address this, I'd like to propose making wal_receiver_timeout
configurable per subscription.

One approach is to add wal_receiver_timeout as a parameter to
CREATE SUBSCRIPTION command, storing it in pg_subscription
so each logical replication worker can use its specific value.

Another option is to change the wal_receiver_timeout's GUC context
from PGC_SIGHUP to PGC_USERSET. This would allow setting different
values via ALTER ROLE SET command for each subscription owner -
effectively enabling per-subscription configuration. Since this
approach is simpler and likely sufficient, I'd prefer starting with this.
Thought?

BTW, this could be extended in the future to other GUCs used by
logical replication workers, such as wal_retrieve_retry_interval.

Regards,

-- 
Fujii Masao
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION