pgsql: Add a xid argument to the filter_prepare callback for output plu

Add a xid argument to the filter_prepare callback for output plugins.

Along with gid, this provides a different way to identify the transaction.
The users that use xid in some way to prepare the transactions can use it
to filter prepare transactions. The later commands COMMIT PREPARED or
ROLLBACK PREPARED carries both identifiers, providing an output plugin the
choice of what to use.

Author: Markus Wanner
Reviewed-by: Vignesh C, Amit Kapila
Discussion: https://postgr.es/m/ee280000-7355-c4dc-e47b-2436e7be959c@enterprisedb.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/f64ea6dc5c8ccaec9a3d3d39695ca261febb6883

Modified Files
--------------
contrib/test_decoding/test_decoding.c | 4 +++-
doc/src/sgml/logicaldecoding.sgml | 29 ++++++++++++++++++-----------
src/backend/replication/logical/decode.c | 17 +++++++++++------
src/backend/replication/logical/logical.c | 5 +++--
src/include/replication/logical.h | 3 ++-
src/include/replication/output_plugin.h | 1 +
6 files changed, 38 insertions(+), 21 deletions(-)