pgsql: postgres_fdw: Allow application_name of remote connection to be

postgres_fdw: Allow application_name of remote connection to be set via GUC.

This commit adds postgres_fdw.application_name GUC which specifies
a value for application_name configuration parameter used
when postgres_fdw establishes a connection to a foreign server.
This GUC setting always overrides application_name option of
the foreign server object. This GUC is useful when we want to
specify our own application_name per remote connection.

Previously application_name of a remote connection could be set
basically only via options of a server object. But which meant that
every session connecting to the same foreign server basically
should use the same application_name. Also if we want to change
the setting, we had to execute "ALTER SERVER ... OPTIONS ..." command.
It was inconvenient.

Author: Hayato Kuroda
Reviewed-by: Masahiro Ikeda, Fujii Masao
Discussion: https://postgr.es/m/TYCPR01MB5870D1E8B949DAF6D3B84E02F5F29@TYCPR01MB5870.jpnprd01.prod.outlook.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/449ab6350526e99d33363706b759951ebad7928e

Modified Files
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contrib/postgres_fdw/connection.c | 25 ++++++--
contrib/postgres_fdw/expected/postgres_fdw.out | 79 ++++++++++++++++++++++++++
contrib/postgres_fdw/option.c | 35 +++++++++++-
contrib/postgres_fdw/postgres_fdw.h | 1 +
contrib/postgres_fdw/sql/postgres_fdw.sql | 39 +++++++++++++
doc/src/sgml/postgres-fdw.sgml | 25 ++++++++
6 files changed, 199 insertions(+), 5 deletions(-)