Add GraphQL query for work item saved views

What does this MR do and why?

Add GraphQL query for work item saved views.

Implement SavedViewsResolver with filtering, search, subscription, and sorting support. Add SavedViewsFinder and model scopes for saved views.

Changelog: added

References

Related to #585074 (closed) #524893 (closed)

How to set up and validate locally

1. Create a saved view and subscribe to it

2. Query all saved views for a namespace:

   query {
     namespace(fullPath: "flightjs") {
       savedViews {
         nodes {
           id
           name
         }
       }
     }
   }

3. Query with filters:

   query {
     namespace(fullPath: "flightjs") {
       savedViews(subscribedOnly: true, search: "My", sort: RELATIVE_POSITION) {
         nodes {
           id
           name
         }
       }
     }
   }

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