[electrum] Sign stakes using Trezor devices

Summary:
This makes it possible to sign stakes with a Trezor device running a compatible firmware.
For now such a firmware doesn't exist unless it's built manually (so it's not official).

Test Plan:
Sign a proof with a Trezor.
Run the tests.

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D17573