Add workItemTypes field to GraphqQL GroupType

What does this MR do and why?

Adds a workItemTypes field to GraphqQL GroupType.

Necessary for work item creation where a workItemTType global ID will be specified. Currently we return all default types that exist on the system, but eventually these types will exist at the root_group level

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 {currentUser { groups { nodes { workItemTypes { nodes { iconName id name } } } } }}
4. Observe the returned work item types

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)