Add getDescendantGroups() methods to GroupApi

[jmini]

Add the methods for this endpoint:

GET /groups/:id/descendant_groups

Docs: https://docs.gitlab.com/ee/api/groups.html#list-a-groups-descendant-groups

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This is similar to getSubGroups() but it returns all child groups in the tree (subgroups of subgroups recursively) as list.

I do not see the point of having all the methods with the different parameters as it was implemented for getSubGroups(), since using a GroupFilter with the builder pattern is better to add only the desired parameters to the request.