Add the ability to query descendant milestones via REST API

What does this MR do and why?

Add support for querying milestones in descendant groups/project. The GraphQL API already supports this, so the relevant code is moved into a shared concern to keep it DRY and provide the same logic for both REST and GraphQL

How to set up and validate locally

1. create a top level group called milestone-test, and a subgroup of it called milestone-subgroup, and a project in the subgroup called milestone-project. Make them both public for ease of testing

2. in each create a milestone

3. open the graphql explorer, such as http://localhost:3000/-/graphql-explorer

4. use the following query

   query {
     group(fullPath: "milestone-test/milestone-subgroup") {
       id
       milestones(includeDescendants: true) {
         nodes {
           id
           title
         }
       }
     }
   }

   You should get the subgroup and project milestones

5. test the REST API, in a new tab enter

   http://localhost:3000/api/v4/groups/84/milestones?include_descendants=true

   where 84 is the group id of the milestone-subgroup group

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #421030 (closed)