Add widget definition fields to work item types query

What does this MR do and why?

Part of !144705 (closed), this MR adds the widgetDefinitions field to workItemTypes query, gives an array of items found on a specific work item.

No user-facing changes.

Adding the widgetDefinitions to mock_data.js was about 1000 lines, and some fields in WorkItemTypeFragment are EE-only.

To save some lines and simplify the EE split, it now uses generated fixtures.

How to test

1. bin/rspec spec/frontend/fixtures/work_items.rb
2. yarn jest work_items

To check FOSS:

1. FOSS_ONLY=1 bin/rspec spec/frontend/fixtures/work_items.rb
2. Compare tmp/tests/frontend/fixtures/graphql/work_items/project_work_item_types.query.graphql.json and tmp/tests/frontend/fixtures-ee/graphql/work_items/project_work_item_types.query.graphql.json for extra fields in the ASSIGNEE block (allowsMultipleAssignees and canInviteMembers)