Hardware (cold) wallet support

[gregdhill]

Is your feature request related to a problem? Please describe.
As a Vault operator I do not feel comfortable holding all of my BTC in a hot wallet which my node can readily access.

Describe the solution you'd like
I should be able to move some fraction of my BTC to a cold wallet.

Describe alternatives you've considered
It may be possible to use MultiSigs for the Vault client to partially sign but this presents many usability and implementation difficulties with external wallets (i.e. Ledger).

Additional context
Without theft reporting it is trivial to transfer BTC out-of-protocol, however the Vault client itself makes some assumptions about the availability of BTC when responding to a redeem request.