Implement the migration process as a standalone GitHub Action

[ahmdmhd]

Currently, the migration process is implemented as a GitHub Actions workflow that users can copy into their own repositories. This works for simple use cases, however, it proves to be complex if the workflow should be updated in the future. A standalone GitHub Action will provide the ability for users to check for the latest changes in the migration process and utilize it in their repository with a simple version change.

Example usage:

- name: Run S2DM migration
  uses: COVESA/s2dm-migration-action@v1
  with:
    repository-path: ./current
    spec-path: ./current/spec
    github-token: ${{ secrets.GITHUB_TOKEN }}
    concept-namespace: ${{ vars.CONCEPT_NAMESPACE || '' }}
    concept-prefix: ${{ vars.CONCEPT_PREFIX || '' }}
    shacl-serialization-format: ${{ vars.SHACL_SERIALIZATION_FORMAT || '' }}
    shacl-shapes-namespace: ${{ vars.SHACL_SHAPES_NAMESPACE || '' }}
    shacl-shapes-prefix: ${{ vars.SHACL_SHAPES_PREFIX || '' }}
    shacl-model-namespace: ${{ vars.SHACL_MODEL_NAMESPACE || '' }}
    shacl-model-prefix: ${{ vars.SHACL_MODEL_PREFIX || '' }}
    skos-namespace: ${{ vars.SKOS_NAMESPACE || '' }}
    skos-prefix: ${{ vars.SKOS_PREFIX || '' }}
    skos-language: ${{ vars.SKOS_LANGUAGE || '' }}

The workflow should also be modified to allow for version changes to be committed to the repository. Additionally, the documentation should be extended to include ways for the users to generate tokens that allow the action to bypass branch protection when committing the version changes.

[ahmdmhd]

Changes introduced in #156 should be integrated after they are merged into main.