[services][identity] add un-signed SIWE message to WalletLoginRequest

Summary: we don't need a walletAddress field but we do need to include the raw message (which contains the address, among other things).

Test Plan: successfully compiled the protos

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: palys-swm, Adrian, atul, karol-bisztyga, yayabosh

Differential Revision: https://phab.comm.dev/D4286