feat: Add retrieve object_tags REST API

[yusuf-musleh]

Description

This PR exposes the retrieve object tag REST API endpoint from oel_tagging.

Supporting Information

• Depends on feat: Implement ObjectTag retrieve REST API openedx/openedx-learning#68

Testing Instructions

• Run your local devstack,
• Make have feat: Implement ObjectTag retrieve REST API openedx/openedx-learning#68 installed locally, to do that:
  • clone the branch to your DEV_STACK/src
  • uninstall your existing openedx-learning package:
    • make lms-shel
    • pip uninstall openedx-learning
  • install the branch you cloned:
    • make lms-shell
    • cd /edx/src
    • pip install -e openedx-learning
  • Check that the endpoint page works:
    • http://localhost:18000/api/content_tagging/v1/object_tags/