Add GUC option to control maximum active replication origins.

Add GUC option to control maximum active replication origins.

This commit introduces a new GUC option max_active_replication_origins
to control the maximum number of active replication
origins. Previously, this was controlled by
'max_replication_slots'. Having a separate GUC option provides better
flexibility for setting up subscribers, as they may not require
replication slots (for cascading replication) but always require
replication origins.

Author: Euler Taveira 
Reviewed-by: Amit Kapila 
Reviewed-by: Masahiko Sawada 
Reviewed-by: Peter Eisentraut 
Reviewed-by: vignesh C 
Discussion: https://postgr.es/m/b81db436-8262-4575-b7c4-bc0c1551000b@app.fastmail.com