Handle Aggref->aggargtypes in out/readfuncs.c

Handle Aggref->aggargtypes in out/readfuncs.c

When communicating with other nodes, we send names of objects instead
of OIDs as those are assigned on each node independently. We failed to
do this for Aggref->aggargtypes, which worked fine for built-in data
types (those have the same OID on all nodes), but resulted in failures
for custom data types (like for example FIXEDDECIMAL).

   ERROR:  cache lookup failed for type 16731

This fixes it by implementing READ/WRITE_TYPID_LIST_FIELD, similarly
to what we had for RELID.

Note: Turns out the WRITE_RELID_LIST_FIELD was broken, but apparently
we never call it in XL as it's only used for arbiterIndexes field. So
fix that too, in case we enable the feature in the future.