Draft: Add source project GraphQL merge request filter

What does this MR do and why?

Related to #385569

This MR adds the ability to filter merge requests by source_project_id.

Screenshots or screen recordings

How to set up and validate locally

1. Find a project with at least 1 merge request (assign it to yourself)
2. Fork the project
3. Create a merge request from the fork (assign it to yourself)
4. Use the following GraphQL query (without the sourceProjectId filter, then filtering on the canonical project id, then the fork project id):

query {
  user(username: "root") {
    assignedMergeRequests(sourceProjectId: 20) {
      nodes {
        id
        sourceProject {
          id
          fullPath
        }
      }
    }
  }
}

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.