Expose the target commit ID through the tag API

What does this MR do?

Exposes a tags target commit ID through the tag API.

Are there points in the code the reviewer needs to double check?

This is my first contribution to GitLab - as such I'd double check everything! That being said, the change is only very small.

Why was this MR needed?

When using annotated tags, there is currently no way to identify the object ID of the annotated tag object itself, only the dereferenced target object that it points to. This MR adds a "target" property to the tag schema and exposes the object ID of the tag object through that. In the case of lightweight tags this will be the same as the "commit" ID, but in the case of annotated tags, this will be the object ID of the tag object itself.

Screenshots (if relevant)

API

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?