de-deduplicate code in DML execution hooks in postgres_fdw

Hi,
While working on a fix related to non-direct DML [1], I noticed that
postgresExecForeignInsert(), postgresExecForeignUpdate() and
postgresExecForeignDelete() functions are almost identical except that
postgresExecForeignInsert() doesn't require ctid. The fix that I was
working is applicable to Delete and Update but can be useful for
Insert as well. I had to add the same code to two places at least and
might have missed fixing one of them. Why don't we have a single
function which prepares the statement, extract parameters, executes
the prepared statement and checks for the results, returned rows etc?
It's been a while that these functions are there and haven't produced
code which is a lot different for each of these cases. Here's a patch
to extract that code into a separate function and use it in all the
three hook implementations.

[1] https://www.postgresql.org/message-id/CAFjFpRfcgwsHRmpvoOK-GUQi-n8MgAS+OxcQo=aBDn1COywmcg@mail.gmail.com

-- 
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company