Add domain field into Clusters::Cluster

What does this MR do?

Add domain into Clusters::Cluster. This is the first step for moving Auto DevOps domain into cluster settings (for project and groups). See https://gitlab.com/gitlab-org/gitlab-ce/issues/52363 for more details.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/52363

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• 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