Add ability to sweep private keys

The Issue

In order to support the LBRY paper wallet, the daemon must allow sweeping/importing of LBC addresses. I'm not sure importing is possible because that collide with being able to import/export passphrases as a backup/restore function.

I'd say this is different from https://github.com/lbryio/lbry/issues/1044, but I could be convinced otherwise. The passphrase export/import would be a backup/restore mechanism for the LBRY wallet

System Configuration

Anything Else

Screenshots