Expose who added me to a group

[nplasterer]

Once xmtp/xmtp-ios#290 and xmtp/xmtp-android#200 land there should be a native function to expose who added me to a group.
Lets expose that function here as well.

• bump to latest pod and gradle
• Create native bridge functions to expose who added me to a group
• Expose a function whoAddedMe on the Group class that returns a address String
• use semantic releases to cut a new npm beta release