Add mutation to create a merge request in GraphQL

What does this MR do?

Issue: #215619 (closed)

GitLab has a REST API to create merge request, but the functionality is not currently available in GraphQL

This MR adds a mutation to GraphQL API that allows to create a new merge request.

Screenshots

An example query for testing

mutation {
 mergeRequestCreate(input: {
    projectPath: "root/project", 
    sourceBranch: "new_branch", 
    targetBranch: "master",
    title: "New Merge Request",
    description: "My description"
  }) {
    mergeRequest {
      title
      createdAt
      description
    }
    errors
  } 
}

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