pgsql: Introduce new extended routines for FDW and foreign serverlooku

Introduce new extended routines for FDW and foreign server lookups

The cache lookup routines for foreign-data wrappers and foreign servers
are extended with an extra argument to handle a set of flags.  The only
value which can be used now is to indicate if a missing object should
result in an error or not, and are designed to be extensible on need.
Those new routines are added into the existing set of user-visible
FDW APIs and documented in consequence.  They will be used for future
patches to improve the SQL interface for object addresses.

Author: Michael Paquier
Reviewed-by: Álvaro Herrera
Discussion: https://postgr.es/m/CAB7nPqSZxrSmdHK-rny7z8mi=EAFXJ5J-0RbzDw6aus=wB5azQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8fb569e978af3995f0dd6b0033758ec571aab0c1

Modified Files
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doc/src/sgml/fdwhandler.sgml  | 34 ++++++++++++++++++++++++++++++++++
src/backend/foreign/foreign.c | 36 ++++++++++++++++++++++++++++++++++--
src/include/foreign/foreign.h | 10 ++++++++++
3 files changed, 78 insertions(+), 2 deletions(-)