pgsql: postgres_fdw: Allow postgres_fdw.application_name to include esc

postgres_fdw: Allow postgres_fdw.application_name to include escape sequences.

application_name that used when postgres_fdw establishes a connection to
a foreign server can be specified in either or both a connection parameter
of a server object and GUC postgres_fdw.application_name. This commit
allows those parameters to include escape sequences that begins with
% character. Then postgres_fdw replaces those escape sequences with
status information. For example, %d and %u are replaced with user name
and database name in local server, respectively. This feature enables us
to add information more easily to track remote transactions or queries,
into application_name of a remote connection.

Author: Hayato Kuroda
Reviewed-by: Kyotaro Horiguchi, Masahiro Ikeda, Hou Zhijie, Fujii Masao
Discussion: https://postgr.es/m/TYAPR01MB5866FAE71C66547C64616584F5EB9@TYAPR01MB5866.jpnprd01.prod.outlook.com
Discussion: https://postgr.es/m/TYCPR01MB5870D1E8B949DAF6D3B84E02F5F29@TYCPR01MB5870.jpnprd01.prod.outlook.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/6e0cb3dec10e460288d68a128e3d79d16a230cdb

Modified Files
--------------
contrib/postgres_fdw/connection.c | 36 ++++++++++++++
contrib/postgres_fdw/expected/postgres_fdw.out | 32 ++++++++++++
contrib/postgres_fdw/option.c | 62 ++++++++++++++++++++++++
contrib/postgres_fdw/postgres_fdw.h | 1 +
contrib/postgres_fdw/sql/postgres_fdw.sql | 21 ++++++++
doc/src/sgml/postgres-fdw.sgml | 67 +++++++++++++++++++++++++-
6 files changed, 218 insertions(+), 1 deletion(-)