release-21.2: importccl: allow changes to referencing set or privileges on UDT

[blathers-crl]

Backport 1/1 commits from #71016 on behalf of @ajwerner.

/cc @cockroachdb/release

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In #69674 we enabled importing of tables which used UDTs. We added the caveat
that these types must not change during the import. In #70987, @ajstorm
uncovered that adding new usages of the type cause an illegal change. This is
a particularly painful limitation as all regional by row tables will use the
enum type of the database. That makes the limitation of import much more
extreme than just precluding renames or modifications of enums or their
members.

To fix this limitation, we permit changes to the referencing set which
occur during the import. We also permit changes to privileges as they
won't impact the correctness of the import.

Relates to #69706
Fixes #70987

Release note (enterprise change): Fixed a limitation of IMPORT for tables
using user-defined types whereby any change to the set of tables or views
which reference the type or any changes to privileges on the type during
the IMPORT would lead to failure. Now new references to the type or GRANT
or REVOKE operations performed while the IMPORT is ongoing will not cause
failure.

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Release justification:

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues.
• Patches should not break backwards-compatibility.
• Patches should change as little code as possible.
• Patches should not change on-disk formats or node communication protocols.
• Patches should not add new functionality.
• Patches must not add, edit, or otherwise modify cluster versions; or add version gates.

If some of the basic criteria cannot be satisfied, ensure that the exceptional criteria are satisfied within.

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters.
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.

Add a brief release justification to the body of your PR to justify this backport.

Some other things to consider:

• What did we do to ensure that a user that doesn’t know & care about this backport, has no idea that it happened?
• Will this work in a cluster of mixed patch versions? Did we test that?
• If a user upgrades a patch version, uses this feature, and then downgrades, what happens?

[cockroach-teamcity]

This change is