add repo-dispatch action with test

[dennisvang]

Added a repo-dispatch action that triggers a repository_dispatch event in another repository via the github API.

This requires a personal access token with "Contents" write permission for the target repository, in the secret-token variable.

See api docs.

Usage example:

jobs:
  e2e:
    needs: publish
    runs-on: ubuntu-latest
    steps:
      - name: trigger e2e tests
        uses: FAIRDataTeam/github-workflows/.github/actions/repo-dispatch@v3
        with:
          secret-token: ${{ secrets.MY_REPO_DISPATCH_TOKEN }}
          target-repo-name: FAIRDataPoint-E2E-Tests
          target-repo-owner: FAIRDataTeam
          event-type: trigger-tests
          client-payload: |
            {
                "fdp_version": "${{ github.ref_name }}",
                "fdp_client_version": "latest"
            }

fixes #14