Implement mutation to create a MR in GraphQL

Problem to solve

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

Further details

Users can POST /projects/:id/merge_requests to create a new merge request.

Required attributes:

| Attribute     | Type           | Required | Description                                                               |
|---------------|----------------|----------|---------------------------------------------------------------------------|
| id            | integer/string | yes      | The ID or URL-encoded path of the project owned by the authenticated user |
| source_branch | string         | yes      | The source branch                                                         |
| target_branch | string         | yes      | The target branch                                                         |
| title         | string         | yes      | Title of MR                                                               |

Proposal

Add a GraphQL endpoint

Documentation

GraphQL is mostly self-documenting.

• https://docs.gitlab.com/ce/api/graphql/
• https://docs.gitlab.com/ee/api/merge_requests.html#create-mr