Add hardware wallet support

[camfairchild]

The wallet should support hardware wallets.
This is the truest security we can offer users from this library.

Hardware wallets offer the best form (if not the only true form of wallet security).
This is done via a UX-flow change in how signing is done by the user.
Using a hardware wallet forces signing to be a necessarily interactive and intentional process.
Completely preventing any issues with malicious wallet clients/software.

Two clients in the initial roadmap are:

• PolkadotVault: #
• Ledger (not completed/blocked)