Add GraphQL read API for merge request saved views

What does this MR do and why?

Adds Types::MergeRequests::SavedViewType and exposes the current user's views as currentUser.mergeRequestSavedViews.

The filters column is exposed as a JSON scalar with its keys camelCased on read, matching WorkItemSavedViewType.

This is the third of four backend merge requests for merge request dashboard saved views:

1. !247083 (merged) - table, model, JSON schema, factory
2. !247090 (merged) - services, finder, and authorization policy
3. This merge request - GraphQL read API
4. !247119 (merged) - GraphQL mutations (create, update, delete)

Example query:

{
  currentUser {
    mergeRequestSavedViews {
      nodes {
        id
        name
        filters
        updatedAt
        userPermissions {
          updateSavedView
          deleteSavedView
        }
      }
    }
  }
}

How to set up and validate locally

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