release-24.3: crosscluster/logical: handle user-defined types in SQL mode

[blathers-crl]

Backport 2/2 commits from #133280 on behalf of @rafiss.

/cc @cockroachdb/release

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This PR also includes a cleanup refactor:

importer,crosscluster: move type resolver to a different package

Since we're using this type resolver for more than just IMPORT, it makes
more sense to put it in a more fitting package.

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crosscluster/logical: handle user-defined types in SQL mode

Previously, using a user-defined type in SQL mode would fail because of
a low level check in the execution engine that verifies that the input
datum has the same type OID as the destination column.

This does not work for LDR, since the input datums come directly from a
source table in a different cluster, so user-defined types have
different OIDs.

This fixes it by setting the datum type to "unknown" before executing
the insert query. The "unknown" type is what is normally used when any
SQL statement is sent to CRDB without explicit type annotations/hints.
When the execution engine sees this type, it will perform an automatic
(and cheap) immutable assignment cast to change the datum to the
appropriate type.

fixes #132164
Release note (ops change): Logical replication streams that reference
tables with user-defined types can now be created with the mode = immediate option.

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Release justification: resolve a GA blocker

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
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[cockroach-teamcity]

This change is