docs/qapi-domain: add resolve_any_xref()

docs/qapi-domain: add resolve_any_xref()

Add the ability to resolve cross-references using the `any`
cross-reference syntax. Adding QAPI-specific cross-reference roles will
be added in a forthcoming commit, and will share the same find_obj()
helper.

(There's less code needed for the generic cross-reference resolver, so
it comes first in this series.)

Once again, this code is based very heavily on sphinx.domains.python.

Signed-off-by: John Snow <jsnow@redhat.com>
Message-ID: <20250311034303.75779-8-jsnow@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>