[native][lib] add registerWalletUser to native IdentityServiceContextProvider

Summary:
we will need this in the useIdentityWalletRegister hook introduced in the next diff

Depends on D11081

Test Plan: flow succeeds. will test this more thoroughly later in the stack when the registerWalletUser method is actually called

Reviewers: ashoat, tomek, inka

Reviewed By: ashoat

Differential Revision: https://phab.comm.dev/D11082