Introduce a new v2 service for chats (#36)

* Initial APIs for sending chat messages with real-time updates

* Add missing proto validation

* Add a placeholder thank you message content

* Define a new v2 Chat service

* Add missing validation to GetMessagesRequest.member_id

* Add missing result codes

* Expand allowable chat v2 cursor byte range

* First pass at a new RPC to start two-way chats, initially from a tip

* Move some ChatMetadata fields into ChatMember

* Update ChatMember field comments

* Add a new chat RPC for revealing identity

* Add IDENTITY_ALREADY_REVEALED to RevealIdentityResponse result

* Small refactor on chat stuff

* Add relevant fields to IdentityRevealedContent

* Add additional comments and a few minor tweaks

* Fill out more comments within the chat models

Author: jeffyanta