Support label list flags and settings

We should add support for comma-separated label list flags (and settings) in [`LabelBuildSettings`](https://cs.opensource.google/bazel/bazel/+/master:src/main/java/com/google/devtools/build/lib/rules/LabelBuildSettings.java), similar to how we have `string_list_flag`.

At this point we shouldn't support `allow_multiple`, see the conversation in comments at https://github.com/bazelbuild/bazel/issues/17477#issuecomment-1474028403

Start with src/main/java/com/google/devtools/build/lib/rules/LabelBuildSettings.java and compare its handling with the existing string_list_flag support. Trace how flags and settings are represented and parsed, then check the linked discussion for the intended limitation. Done means comma-separated label list flags and settings are supported without allow_multiple.

Written by the indexing model from the issue text.