[native] logic for navigating to ExistingEthereumAccount if usingCommServicesAccessToken

Summary:
use FindUserID RPC to check if wallet address is associated with existing account

Depends on D11005

Test Plan: successfully navigated to ExistingEthereumAccount with usingCommServicesAccessToken set when trying to register with my already-registered eth wallet

Reviewers: ashoat, inka, tomek

Reviewed By: ashoat

Subscribers: will

Differential Revision: https://phab.comm.dev/D11016