[lib] add identity register action

Summary:
since this action is native-specific, i've created a new interface for native-only client methods.

Depends on D10440

Test Plan: successfully dispatched the action on native and registered a new account with staging identity service

Reviewers: ashoat, tomek, inka

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D10454