importer: fix collated string imports for MySQL/DELIMITED DATA

[otan]

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.

[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 @otan)

[otan]

thanks

bors r=drewkimball

[craig]

Build failed:

• Bazel Essential CI (Cockroach)

[otan]

flakes

#108238
#108256

[otan]

bors r=drewkimball

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)

[blathers-crl]

Encountered an error creating backports. Some common things that can go wrong:

1. The backport branch might have already existed.
2. There was a merge conflict.
3. The backport branch contained merge commits.

You might need to create your backport manually using the backport tool.

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error creating merge commit from d69fe91 to blathers/backport-release-22.2-107918: POST https://api.github.com/repos/cockroachdb/cockroach/merges: 409 Merge conflict []

you may need to manually resolve merge conflicts with the backport tool.

Backport to branch 22.2.x failed. See errors above.

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