[CI Minutes] Create a GraphQL local resolver for "create CI Minutes plan"-call

Create a GraphQL local resolver to integrate the "create CI Minutes plan"-call into the GraphQL:

• See comment here about using Subscriptions::CreateService.new(...): gitlab#300148 (comment 504397238)
• Using the /-/subscription-endpoint that hits subscriptions_controller#create

Details in the Epic: &5391 (closed)