Add ability to apply shared configs from another repo

[narwold]

We had a use case where we wanted to apply the same label configs to multiple repos so that we could have some cross-cutting concerns across our repos. We stored these configs in our .github repo, but we needed a way to apply them without copy-pasting to all our other repos. This PR adds the ability to apply a shared-configurations option like this:

name: "Pull Request Labeler"
on:
- pull_request_target

jobs:
  triage:
    runs-on: ubuntu-latest
    steps:
    - uses: actions/labeler@main
      with:
        repo-token: "${{ secrets.GITHUB_TOKEN }}"
        shared-configurations: '["my-org/my-repo/some-path/shared-preset.yml@branchname", "my-org/my-repo/some-path/another-preset.yml@branchname"]'