release-23.1: importer: fix collated string imports for MySQL/DELIMITED DATA

[blathers-crl]

Backport 1/1 commits from #107918 on behalf of @otan.

/cc @cockroachdb/release

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We previously special cased ParseDatumStringWithRawBytes for MySQL related imports, which was buggy for collated strings as well. Instead, make import have a specialised method for converting MySQL literals to the relevant data type without compromising on collated strings.

Resolves #107917

Release note (bug fix): Previously, using IMPORT INTO for DELIMITED DATA or MySQL imports would error with column ... does not exist if it was importing into a collated string column.

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Release justification: fixes bug with importing mysql data

[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 

[DrewKimball]

Reviewed 5 of 5 files at r1, all commit messages.
Reviewable status: complete! 1 of 0 LGTMs obtained (waiting on @blathers-crl[bot] and @otan)

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pkg/sql/importer/read_import_mysql.go line 214 at r1 (raw file):

		//  mysql emits raw byte strings that do not use the same escaping as our ParseBytes
		//	function expects, and the difference between ParseStringAs and
		//	ParseDatumStringAs is whether or not it attempts to parse bytes.

[nit] I just noticed, I think this comment is stale.