[hooks] Add baseUri to user defines

[simolus3]

Description

This adds baseUriForDefine to HookInputUserDefines`, which returns a base URI that can be used to interpret arbitrary configuration values as paths.

Additionally, it accepts a list of keys, which allows traversing through nested options (yaml maps and sequences). Path elements are either strings (used to lookup values in maps) or integers (for lookups in sequences).

This is a generalization of HookInputUserDefines.path, which allows interpreting a top-level user-define as a path. baseUriForDefine also works for nested paths and can be used to transform lists, maps or any other structure a hook might read as user-defines.

Related Issues

Fixes #3515

PR Checklist

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.
• I've run dart tool/ci.dart --all locally and resolved all issues identified. (Too lazy, I just ran the formatter and checked for analysis errors)
• All existing and new tests are passing. I added new tests to check the change I am making.
• The PR is actually solving the issue. PRs that don't solve the issue will be closed. Please be respectful of the maintainers' time. If it's not clear what the issue is, feel free to ask questions on the GitHub issue before submitting a PR.
• I have updated CHANGELOG.md for the relevant packages. (Not needed for small changes such as doc typos).
• I have updated the pubspec package version if necessary.

[dcharkes]

Lol, now accessing baseUri is easier than the user-defines themselves 😄

[simolus3]

I can add a helper that traverses through options and returns the original define, but I suspect most hooks with complex options would use helper packages like json_serializable for this anyway.

[dcharkes]

@goderbauer See related bug. Any preferences for an API?

[simolus3]

The changelog check fails because it expects a hooks_runner entry due to the test_data/ change, could I get a skip-changelog-check label for this?