[Identity] Add deleteUser RPC to proto files

Summary:
Add deleteUser logic to identity service.

Depends on D6787

Part of https://linear.app/comm/issue/ENG-2764

Test Plan:

cargo build

Gets end-to-end tested in a later diff

Reviewers: varun, max, bartek

Reviewed By: varun, bartek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6788