[native] add reserved registration methods to IdentityServiceContextProvider

Summary:
we need this so we can call the methods from user-actions.js using the identity client

Depends on D11909

Test Plan: tested from js later in stack by calling the useLogIn hook with a reserved username and reserved wallet address and confirming that i was registered on identity

Reviewers: bartek, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11910