GitLab Migration - validate input data before starting a migration

From https://gitlab.com/gitlab-org/manage/import/discussions/-/issues/30#note_1183146440

GitLab Migration allows user to enter a list of import entities with source full path as URL, e.g.

      {
        "source_full_path": "http://gitlab.example/gitlab-org/manage/import/gitlab-migration-test",
        "source_type": "group_entity",
        "destination_slug": "gitlab-migration-test",
        "destination_namespace": "import-test"
      }

But we expect each value in a certain format, e.g. source_full_path has to be a relative full path, not a URL.

Proposal

• Validate each of the fields to return a clear error if user entered something incorrectly.
• In addition to the source_full_path validation it'd make sense to add a few more:
  • Similarly to source_full_path, destination_namespace should be validated in the same way
  • Should the value of destination_slug be validated in the same way as group/project slug is validated?
  • While we're at it, I wonder if it makes sense to add user permission checks here, too. We do have permission checks later (e.g. in GroupLoader) but that's when migration is already in progress