pgsql: Don't include execnodes.h in replication/conflict.h

Don't include execnodes.h in replication/conflict.h

... which silently propagates a lot of headers into many places
via pgstat.h, as evidenced by the variety of headers that this patch
needs to add to seemingly random places.  Add a minimum of typedefs to
conflict.h to be able to remove execnodes.h, and fix the fallout.

Backpatch to 18, where conflict.h first appeared.

Discussion: https://postgr.es/m/202509191927.uj2ijwmho7nv@alvherre.pgsql

Branch
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REL_18_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/47da1745244b25ba8b9396326d7f36e4ed1fb701

Modified Files
--------------
src/backend/access/transam/multixact.c      |  1 +
src/backend/access/transam/xlogrecovery.c   |  1 +
src/backend/storage/ipc/waiteventset.c      |  1 +
src/backend/utils/activity/pgstat_backend.c |  1 +
src/include/pgstat.h                        |  1 +
src/include/replication/conflict.h          | 24 ++++++++++++++++--------
6 files changed, 21 insertions(+), 8 deletions(-)