Add mutation to accept merge requests

What does this MR do?

This adds a GraphQL mutation to accept merge requests.

See:

• #217475 (closed)
• #32665

Example queries

mutation {
  mergeRequestAccept(input: { projectPath: "gitlab-org/gitlab", iid: "54758", strategy: "MERGE_TRAIN", sha: "deadbeef" }) {
    errors
    mergeRequest { status }
  }
}

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

This exposes a feature via HTTP APIs that is currently only available via controller actions. That makes it more available for abuse. This is only available to logged in users with maintainer permissions, so this risk is minimal.

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

Security

This adds a new policy rule, which replicates the logic found in the controller and API resources. This is not a change in authorization.

• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods