[Protos] Add UpdateUser definitions

Summary:
gRPC definition of updateUser protocol

This is primarily meant to garner feedback, while
I still work implmenting the vertical slice with
keyserver.

Notable differences from RegisterUser is that the
final PAKE Register message and first PAKE Login messages are kept
separate. This is done to separate concerns around
the two PAKE actions for https://linear.app/comm/issue/ENG-3149

https://linear.app/comm/issue/ENG-2763

Test Plan:
This is mostly intended to be protocol oriented

cd services/identity
cargo build

Reviewers: varun, max, ashoat

Reviewed By: varun

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6912