Add `include_descendants` parameter to Group Milestones API

Release notes

Group Milestones API doesn't return the milestones for the subgroups and projects under the group.

Possible solution is to add an additional parameter called include_descendants to the API call, similar to the GraphQL API: !30850 (merged)

Problem to solve

Customer is requesting a feature to include the milestones of all the subgroups and projects under the group when using the Group Milestones API.

ZD link: https://gitlab.zendesk.com/agent/tickets/177403 (internal)

Intended users

User experience goal

Proposal

Further details

Permissions and Security

Documentation

Availability & Testing

What does success look like, and how can we measure that?

What is the type of buyer?

Is this a cross-stage feature?

Links / references