Sign bitcoin transactions using integrated 'Sign with Google'

[mgravitt]

NBV currently works with Bluewallet, which requires an install, and that creates a point of friction for new users just getting started.

To eliminate this friction point, the user should be able to authenticate with Google (or private-server?) to save bitcoin keys and then use those keys in the browser to sign transactions.

• Ability to persist bitcoin keys securely with Google Drive API
• UI components for importing and/or generating bitcoin keys
• Add bitcoin signing flow screens to transaction proposal screens
• Add bitcoin signer approval dialog