Include cluster domain in Project Clusters API

What does this MR do?

Include cluster domain into API

Domain was introduced on 11.8 and was not included on the Project Cluster API. With this change, user will be able to include domain when adding and updating a cluster. Domain will also be included on the GET calls.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/59441

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• [-] Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• [-] Security reports checked/validated by reviewer

Closes #59441 (closed)