A new C function `get_partition_root`.

This patch introduces a new C function `get_partition_root`.

WHY?

1. It is the C equivalent of the already-existing SQL function
`pg_partition_root`

2. Some code is not interested in the partition "ancestors", yet it
still had to declare/fetch/free an ancestors list to obtain the root
relid at llast_oid(ancestors).

3. The logical replication FOR TABLES IN SCHEMA EXCEPT (...) patches
[1] (still in development) have many more code fragments like this
where the logic wants only the partition root OID -- This function
would simplify all those.

~~~

PSA v1

Passes make check-world.

Thoughts?

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[1] https://www.postgresql.org/message-id/flat/CABdArM5sw4Q1ZU8HGdo4BSc1A_%2B8xtUNq17j6wcir%3DyMUy19Cg%40mail.gmail.com

Kind Regards,
Peter Smith.
Fujitsu Australia