Add milestone query builder

What does this MR do and why?

Related to #462684 (closed)

Implement a new query builder for milestones scope

Behind a disabled by default feature flag

Add a new search filter for type. Milestones are not in a separate index (like other document types), so the type field must be queried

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

N/A

How to set up and validate locally

pre steps

1. setup gdk for elasticsearch and index the instance
2. test these steps with the feature flag on and off
3. verify the results are the same

testing

1. create a public project with 1 milestone named "public milestone"
2. create a private project with 1 milestone named "private milestone"
3. create an internal project with 1 milestone named "internal milestone"
4. create a public project with 1 milestone named "public disabled feature milestone", disable merge requests and issues for the project
5. give developer access to the group to a user
6. login as that user
7. run searches and note results

• global: http://gdk.test:3000/search?scope=milestones&search=milestone
• group: http://gdk.test:3000/search?scope=milestones&search=milestone&group_id=1055
• project: http://gdk.test:3000/search?scope=milestones&search=milestone&group_id=1055&project_id=73

8. load search up using an anonymous user
9. run the same searches and note results

Repeat the above with the feature flag enabled and verify search results are the same