[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