Add getGroupTangle

[Powersource]

based on #2

We need this to add correct group tangles to group messages

• probably combine addGroupTangle and create into a function with better dx, that we then probably expose in the top level api

[arj03]

It would be good with a test of how this performs if you have 10.000 messages in a group

[staltz]

@arj03 Could perf be studied and optimized in a different PR?

[arj03]

Yes, its just so that we:

1. know how it performs in realistic settings
2. can use this as a baseline for optimizations (and make sure we don't make things slower :-))

[staltz]

Thanks for the changes!