[identity] introduce RegisterWalletUser RPC

Summary:
the existing LogInWalletUser RPC now handles login exclusively. Thew new Register RPC will handle registration exclusively. Changes in this diff:

• Introduced new RPC in .proto file and renamed WalletLoginRequest -> WalletAuthRequest so we can use it for both registration and login
• Implemented the new register RPC on the identity service, and updated the existing login RPC to only handle login
• Updated grpc-web codegen to reflect the .proto changes
• Updated the native_rust_library to use the new WalletAuthRequest message name

Depends on D11062

Test Plan: confirmed that web flow check passes and native_rust_library and identity build successfully. will test the wallet RPCs further in the stack starting with D11005

Reviewers: ashoat, bartek

Reviewed By: ashoat, bartek

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11078