[keyserver] call Identity service RegisterUser RPC from createAccount

Summary:

• fixed the registerUser function type in lib
• call registerUser from createAccount if the signedIdentityKeysBlob is present

Depends on D6914

Test Plan:

• first, I created a new account on mobile with username/password; saw that the correct user data was in the user and token tables in DDB
• second, logged out and back in on web; nothing changed in users table
• third, logged in on web, saw that a new device was added to the users table and a new access token was added to the tokens table

also tested that if we're disconnected from the identity service for whatever reason, nothing crashes on the keyserver

Reviewers: jon, ashoat, bartek, atul

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6941