feat: use work items api for epics

What does this MR do and why?

feat: use work items api for epics

Force FF glqlWorkItems = true for epics, but pass a constraint excludeProjects: true that ensures that performance of the API is at par with legacy epics API, but supports all new work items features

Why: While work items API performance isn't at par with issues, work items API for epics has equivalent performance now with legacy issues if we pass excludeProjects: true in that API. I've changed the epic source to force enable work items FF and add that constraint to the API.

How to set up and validate locally

1. Refer to the end to end testing guide section in README.md.
2. Ensure that the feature flag glql_work_items is turned off.
3. In GDK, insert a GLQL query on a page with query type = Epic.
4. Notice that a request to work items is sent on the backend not legacy epics.