Support loading of GitLab `!reference` data during validation

Preface

GitLab !reference support was added in https://github.com/python-jsonschema/check-jsonschema/issues/112 but there are still some edge cases.

How I verified that there are no duplicate issues in the issue tracker

I searched for the "reference" word between open and closed issues and I found only closed https://github.com/python-jsonschema/check-jsonschema/issues/112.

.pre-commit-config.yaml

  - repo: https://github.com/python-jsonschema/check-jsonschema
    rev: 0.23.1
    hooks:
      - id: check-gitlab-ci
        args: ["--data-transform", "gitlab-ci"]

Valid .gitlab-ci.yml

include:
  - project: "myproject/mynamespace/backend/repositoryname"
    ref: v1.1.22
    file: ".gitlab-ci-common.yml"

test:
  stage: test
  services:
    - name: mongo:6.0.6
      command: ["/bin/sh", "-c", "mongod --logpath /dev/null --bind_ip_all --replSet 'rs0' "]
    - !reference [.common_services_for_test_job, services]
  before_script:
    - echo "Hello"

check-gitlab-ci output

Validate GitLab CI config................................................
Failed
- hook id: check-gitlab-ci
- exit code: 1
Schema validation errors were encountered.
  .gitlab-ci.yml::$.test.services[1]: ['.common_services_for_test_job', 'services'] is not valid under any of the given schemas
  Underlying errors caused this.
  Best Match:
    $.test.services[1]: ['.common_services_for_test_job', 'services'] is not of type 'string'

Expected result

Accept the mentioned YAML syntax.

Start by reproducing the failure with the check-gitlab-ci hook and --data-transform gitlab-ci against the YAML example in the issue. Trace the validation path for the services entry and determine how GitLab !reference values are represented during loading. Done means the shown syntax is accepted without the schema validation error and existing validation still passes.