Implement generic createWorkItem mutation

Problem to solve

We need a way to create Work Items via GraphQL. Specifically, this is required for the creation of Tasks, but should be allow for the creation of any work item type.

Proposal

• Add GraphQL mutation for creating an issue of type Task (!73275 (diffs))
• The mutation should validate that a title and type are specified.

Acceptance

• The mutation successfully creates a new issue of type Task related to the issue from which it was created.
• Creating a task increments g_project_management_issue_created_monthly