[Identity] Implment update_user endpoint on server

Summary:
Most of the registration code was taken and adapted to
updating a user. A small amount of rust refinement was added to
make the code a bit more readable.

Updates the user's public signing key, password (registration_bundle),
and will issue a new access token.

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

Depends on D6925

Test Plan:
will be tested end-to-end in another diff

cd services/identity
cargo run -- server

Reviewers: varun, bartek, max

Reviewed By: varun

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6939