WalletConnect / external signer support

[mhluongo]

As a precursor to more hardware wallet support, we plan to support WalletConnect as an external signer.

This looks a little different than regular WalletConnect support (which we also need, less urgently).

Any user should be able to add a WalletConnect account to the extension. The extension should show balances, incoming transactions, etc the way it would for other accounts, and outsource transaction signing to the connected wallet.