pgsql: Add copy/equal support for XID lists

Add copy/equal support for XID lists

Commit f10a025cfe97 added support for List to store Xids, but didn't
handle the new type in all cases. Add some obviously necessary pieces.
As far as I am aware, this is all dead code as far as core code is
concerned, but it seems unacceptable not to have it in case third-party
code wants to rely on this type of list. (Some parts of the List API
remain unimplemented, but that can be fixed as and when needed -- see
lack of list_intersection_oid, list_deduplicate_int as precedents.)

Discussion: https://postgr.es/m/20220708164534.nbejhgt4ajz35p65@alvherre.pgsql

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5ca0fe5c8ad7987beee95669124c7e245f2816d8

Modified Files
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src/backend/nodes/copyfuncs.c | 5 +++--
src/backend/nodes/equalfuncs.c | 8 ++++++++
src/test/modules/test_oat_hooks/test_oat_hooks.c | 3 +++
3 files changed, 14 insertions(+), 2 deletions(-)