Make get_extension_schema() available

Make get_extension_schema() available

This routine is able to retrieve the OID of the schema used with an
extension (pg_extension.extnamespace), or InvalidOid if this information
is not available.  plpgsql_check embeds a copy of this code when
performing checks on functions, as one out-of-core example.

Author: Pavel Stehule
Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/CAFj8pRD+9x55hjDoi285jCcjPc8uuY_D+FLn5RpXggdz+4O2sQ@mail.gmail.com