Refactor dashboard ServiceSelector to use consistent selection API

What does this MR do?

This refactor aligns how a dashboard service is selected. All dashboard services should now have a ::valid_params? class method which can be used to determine if the params are sufficient to potentially find/create a dashboard using that service.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers