pgsql: Make logical decoding a part of the rmgr.

Make logical decoding a part of the rmgr.

Add a new rmgr method, rm_decode, and use that rather than a switch
statement.

In preparation for rmgr extensibility.

Reviewed-by: Julien Rouhaud
Discussion: https://postgr.es/m/ed1fb2e22d15d3563ae0eb610f7b61bb15999c0a.camel%40j-davis.com
Discussion: https://postgr.es/m/20220118095332.6xtlcjoyxobv6cbk@jrouhaud

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/7a5f6b47488d824b1ea1326be4337e2c32325ff2

Modified Files
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src/backend/access/transam/rmgr.c | 5 +-
src/backend/replication/logical/decode.c | 105 +++++++------------------------
src/bin/pg_rewind/parsexlog.c | 2 +-
src/bin/pg_waldump/rmgrdesc.c | 2 +-
src/include/access/rmgr.h | 2 +-
src/include/access/rmgrlist.h | 44 ++++++-------
src/include/access/xlog_internal.h | 5 ++
src/include/replication/decode.h | 16 ++++-
8 files changed, 69 insertions(+), 112 deletions(-)