Update pg layer for general fdw use

Building on TEIID-5498 we should allow general use of fdw access. This is much broader in determining what function and type support is expected. At the very least type casting needs to be aware of the pg type names as a literal like 'a' will be pushed as 'a'::text.

At this time since joins and other higher level constructs are not pushed that still keeps the scope somewhat narrow.