CREATE SUBSCRIPTION ... SERVER.

CREATE SUBSCRIPTION ... SERVER.

Allow CREATE SUBSCRIPTION to accept a foreign server using the SERVER
clause instead of a raw connection string using the CONNECTION clause.

  * Enables a user with sufficient privileges to create a subscription
    using a foreign server by name without specifying the connection
    details.

  * Integrates with user mappings (and other FDW infrastructure) using
    the subscription owner.

  * Provides a layer of indirection to manage multiple subscriptions
    to the same remote server more easily.

Also add CREATE FOREIGN DATA WRAPPER ... CONNECTION clause to specify
a connection_function. To be eligible for a subscription, the foreign
server's foreign data wrapper must specify a connection_function.

Add connection_function support to postgres_fdw, and bump postgres_fdw
version to 1.3.

Bump catversion.

Reviewed-by: Ashutosh Bapat <ashutosh.bapat.oss@gmail.com>
Reviewed-by: Shlok Kyal <shlok.kyal.oss@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Discussion: https://postgr.es/m/61831790a0a937038f78ce09f8dd4cef7de7456a.camel@j-davis.com