Adds workItemCreate mutation to the GraphQL API

What does this MR do and why?

Adds workItemCreate mutation to the GraphQL API. This first iteration is intended to create task work item types. A separate MR will provide the query to list existing work item types.

How to set up and validate locally

1. Run bundle exec rails console
2. Run Feature.enable(:work_items)
3. Execute a GraphQL query like

mutation {
  workItemCreate(input: {projectPath: "Flightjs/flight", title: "test", workItemTypeId: "gid://gitlab/WorkItems::Type/2"}) {
    workItem {
      title
      workItemType {
        id
        name
      }
    }
  }
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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

Related to #346041 (closed)