release-22.2: sql/catalog/schemaexpr: fix default expression assignment casts

[mgartner]

Backport 1/1 commits from #95459.

/cc @cockroachdb/release

---

In #95398, casts were added to default expressions during backfills when
the default expression's type was not equivalent to the target column's
type. This commit fixes two minor bugs with the previous fix:

1. Casts were only added when types were not equivalent, i.e., when
   they had different families. Assignment casts must be added for
   non-identical types with the same family. For example, a TEXT
   default expression must be assignment cast to a CHAR(1) column,
   even though they are both in the string family.
2. Explicit casts were added instead of assignment casts, which have
   different behavior. For example, an assignment cast will error when
   casting TEXT to CHAR(1) if the string has a length greater than
   one.

Some tests were added to show that default column backfills behave
consistently with Postgres.

Informs #93398

Epic: None

Release note: None

Release justification: Amendment to a recently merged backport.

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Patches should only be created for serious issues or test-only changes.
• 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 

[ajwerner]

thanks again

Reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @mgartner)