Add postgres_fdw contrib module.

There's still a lot of room for improvement, but it basically works,
and we need this to be present before we can do anything much with the
writable-foreign-tables patch.  So let's commit it and get on with testing.

Shigeru Hanada, reviewed by KaiGai Kohei and Tom Lane

doc/src/sgml/contrib.sgml

@@ -132,6 +132,7 @@ CREATE EXTENSION <replaceable>module_name</> FROM unpackaged;
  &pgstatstatements;
  &pgstattuple;
  &pgtrgm;
+ &postgres-fdw;
  &seg;
  &sepgsql;
  &contrib-spi;