Re: explain plans for foreign servers

Thanks a lot Solai for testing the patch, appreciate it.

Hi Bharath,
Thanks for the inputs, attached v7 patch to make review easier.

> IMO, we don't need to print all the flavors of explain plans
I have removed formatting related tests from file postgres_fdw.sql and
moved to a separate file remote_plans_formats.sql.
We still need tests around formatting as my changes introduce some new
elements.

> Another point, why are we not supporting explain analyze?
This will be part of follow-up work, I wanted agreement on the shape first
from the community.
One design I considered is an auto_explain-style GUC asking the remote to
emit the plan for the statement it is already
running, but we cannot assume an arbitrary foreign server has that, so it
needs a fallback and more thought.

> 1/ Why not get the extension id at once in _PG_init
Fixed, also I moved all the EXPLAIN plumbing into postgres_fdw.c behind
postgres_fdw_explain_init(), called from
_PG_init. The extension id, the saved hook pointers and both hook
functions are now static.

Addressed other comments regarding documentation around GENERIC_PLAN option
and added error case handling non-existent remote table.