builtins: implement to_char(date, string)

[otan]

Release note (sql change): Introduce the to_char(date, format) builtin variant, which converts a given date to a string using the given format string.

Informs #3781

(if we had implicit casts this wouldnt be an issue!)

[cockroach-teamcity]

This change is 

[otan]

bors r=rafiss

[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 7a4bbaa to blathers/backport-release-22.2-102787: 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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error creating merge commit from 7a4bbaa to blathers/backport-release-23.1-102787: 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 23.1.x failed. See errors above.

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