BE implementation - Implement GraphQL mutation for MR Creation flow

A Rest API has been implemented in #223878 (comment 376076045). According to GitLab's value on GraphQL everywhere, we need to convert the Rest API to corresponding GraphQL mutation.

Implementation Plan

• Create a GraphQL mutation in ee/app/graphql/ee/types
• Follow the MR to see what changes are made in MR creation issue and use the model/service that is used for creating MR.