Backend: Move pipeline header to GraphQL

This is the backend issue for: #223261 (closed). In the spirit of iteration and efficiency, it will probably be done in multiple MRs.

Query: The user

{ 
  pipeline(id){
    user{
       ...fields of the user that triggered the pipeline
    }
}

Mutations:

• Retry pipeline by ID
• Cancel pipeline by ID
• Delete pipeline by ID