Return the Public Key of the account used during Login as part of the LoginResult

[aaroncox]

Relevant conversation on Telegram starting here:

https://t.me/wharfkit/1163

Research will be needed to figure out an appropriate path to return this data, preferably without extra API calls.

[aaroncox]

As a step forward in this, a WalletPlugin may optionally now return an IdentityProof that's made available to the application. This isn't a hard requirement though, and not all wallet plugins will sign transactions during the login process.