[7.4.0] Add a string-to-label-dict attribute type.

[lberki]

This really just exposes the existing LABEL_DICT_UNARY type that already exists in Java.

Other than writing some boring conversion logic, all pieces fell together pretty pleasantly.

Work towards #7989. That one calls for a string-to-label-list-dict type, but I'm planning to resolve that as WAI, since string-to-label-dict is close enough.

RELNOTES: None.
PiperOrigin-RevId: 686415025
Change-Id: Ib07ada7ab2ede95220ed1cc2d3569996fc4afb88

[meteorcloudy]

How important is this? We are already at rc2 for 7.4.0, we would like to push back non critical changes and avoid delaying the release further.

[lberki]

Not very, I'd say. I sent out this pull request spurred by @fmeum 's comment #7989 (comment) that if we don't backport this to 7.4.0, it'll take a much longer time for rule sets to make use of this new attribute type. It wasn't complicated, so I thought it was worthwhile but I was not aware of the release schedule of 7.4.0.

[meteorcloudy]

it'll take a much longer time for rule sets to make use of this new attribute type

Hmm, this sounds like it's probably worth another rc.

[tetromino]

FYI: unfortunately this cherry-pick was done without cherry-picking related updates to AttributeInfoExtractor.java, so Stardoc + Bazel 7.4/5 will fail on .bzl file using attr.label_keyed_string_dict() :/

See bazelbuild/stardoc#277

[fmeum]

7.5.0 is still open

[Wyverald]

Indeed; I would prefer that we take this chance to cherry-pick a proper fix, as 7.5.0 is still waiting on the merge of a separate PR, and will have another RC next Monday.