Implement 1:1 Chats using Groups

[jazzz]

Problem

Currently 1:1 chats do not support HNDL protections via post-quantum encryption. Updating them would be be costly for limited benefit a this point.

Whats Needed

• Create PrivateV2Convo which is based on groups using InboxV2
• Limit membership to two participants
• Update create_private_convo to initialize PrivateV2Convo

Notes

• Exploring PQ 1:1 chats should be explored at a later point due to privacy implications
  • Currently lacking PQ:HNDL is a bigger issue.
• Verification needed: It should be possible to use base openMLS (Implemented In GroupV1) in a two person group. De-MLS adds consistency for multi-party state updates, however the 2 participant case is less complex.