Create cluster annotations API endpoint

What does this MR do?

Resolves #211460 (closed), implementing an endpoint to create metrics dashboard annotations attached to a cluster. Builds off of previous MR to create annotations off of an environment: !29089 (merged) (currently in review, will change base branch back to master after it is merged)

POST /clusters/:cluster_id/metrics_dashboard/annotations/

Required parameters:

• dashboard_path: string, The ID of the dashboard which needs to be annotated
• starting_at: Date time string, ISO 8601 formatted, e.g. 2016-03-11T03:45:40Z. Timestamp marking start point of annotation
• description: string, Description of the annotation

Optional parameters:

• ending_at: Date time string, ISO 8601 formatted, e.g. 2016-03-11T03:45:40Z. Timestamp marking end point of annotation.

Documentation MR: !29508 (merged)

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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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #211460 (closed)