API SDK: Have a way to create a group using the members of other groups

[vplasencia]

Description

It would be nice to combine groups, create new groups that are unions of existing groups.

Like group 1 and 2 would already have members.

There would be eg a new function: createGroupFrom({name, desc, fingetPrint, depth}, groupId[], apiKey) that probably just needs to call existing methods of the sdk: createGroup, then loop over the groupIds to do getGroup and addMembersByApiKey

This would construct a group that is already full of the members of group 1 and 2. And therefore the new group benefits immediately from the anonymity sets of the 2 others.

proposed by @sripwoud