This commit performs some refactoring for the way data source objects
are tracked in `opt.Metadata` in order to make future changes to UDT
and UDF dependency tracking easier. More particularly, UDTs and UDFs
will be able to re-resolve any references by name. This is necessary
in order to handle cases where changes to the search-path cause names
in the query to resolve to different objects.

Release note: None

Previously, it was possible for invalid queries to be kept in the cache
after a schema change, since user-defined types were tracked only by OID.
This missed cases where the search path changed which object a name in
the query resolved to (or whether it resolved at all).

This patch fixes this behavior by tracking UDT references by name, similar
to what was already done for data sources. This ensures that the query
staleness check doesn't miss changes to the search path.

Fixes cockroachdb#96674

Release note (bug fix): Fixed a bug that could prevent a cached query with
a user-defined type reference from being invalidated even after a schema
change that should prevent the type from being resolved.

This patch adds tracking for user-defined functions in `opt.Metadata`.
This ensures that the query cache will be correctly invalidated after
a schema change or search-path change that could change the query's
semantics, or cause it to error.

Fixes cockroachdb#93082
Fixes cockroachdb#93321

Release note (bug fix): Fixed a bug that could prevent a cached query
from being invalidated when a UDF referenced by that query was altered
or dropped.

It is possible to define a user-defined function with the same signature
as a builtin function. Normally, an unqualified function call will resolve
to the builtin function because its schema will be first in the search path.
However, it is possible to modify the search path, so that the same function
call can resolve to different functions on different executions. Example:
```
CREATE FUNCTION public.abs(val INT) RETURNS INT CALLED ON NULL INPUT LANGUAGE SQL AS $$ SELECT val+100 $$;
SELECT abs(1); --This should resolve to the builtin abs().
SET search_path = public, pg_catalog;
SELECT abs(1); --This should resolve to the udf abs().
```

Fixes cockroachdb#97757

Release note (bug fix): Fixed a bug existing from when user-defined
functions were introduced that could cause a function call to resolve
to the wrong function after changes to the schema search path.