Patches to make pg_dump compatible with FDW

These patches extend the compatibility of pg_dump with foreign data wrappers for PostgreSQL 13 (it can easily be adapted for Postgres versions 11 and 12). Now data in FDW backed tabled can be optionally dumped and the ALTER TABLE commands on foreign tables are written as ALTER FOREIGN TABLE.

To compile and use the patch follow these instructions:

1. Get the PostgreSQL source code, for example via
   • git clone https://github.com/postgres/postgres.git --depth=1 or
   • apt-get source postgresql-11 and sudo apt-get build-dep postgresql-11
2. Apply these patches with e.g.: patch -p1 < somepatch
3. Compile PostgreSQL

The related threads in the PostgreSQL hackers mail list are:

• https://www.postgresql.org/message-id/flat/CALDaNm1pdQHgp14ppcUkzQ7AQPodEyBCbzbkK6%2BuETxn0TKpQA%40mail.gmail.com#6c7457d32
• https://www.postgresql.org/message-id/flat/20200114230411.GA10233%40alvherre.pgsql#6d0ed361f0c99be940f53dad86db6839