Add the ability to query descendant iterations via REST and GraphQL API

What does this MR do and why?

Adds support for retrieving descendant iterations from a group, in both GraphQL and REST.

How to set up and validate locally

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

2. in each group create cadence (Plan -> Iterations), non-automatic. Then for each cadence create a new iteration (3-dot menu to the right of the cadence list)

3. open the graphql explorer, such as http://ee.gitlab.test:5100/-/graphql-explorer

4. use the following query

   query {
     group(fullPath: "iteration-test") {
       id
       iterations(includeDescendants: true) {
         nodes {
           createdAt
           title
         }
       }
     }
   }

5. test the REST API,

   curl http://ee.gitlab.test:5100/api/v4/groups/76/iterations\?include_descendants\=true

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 #430255 (closed)