Support transaction creation in response to a ZIP 321 payment request.

[nuttycom]

Is your feature request related to a problem? Please describe.

ZIP 321 is the standard that describes how merchants should request Zcash payments. Zashi should support scanning ZIP 321 QR codes.

Describe the solution you'd like

When I scan a ZIP 321 QR code, my wallet should attempt to construct a transaction proposal to fulfill the request, and if successful should take me directly to the send confirmation screen.

Alternatives you've considered

At present, YWallet registers as a device-wide handler for ZIP 321 URIs. We should consider doing this as well; otherwise a user scanning a ZIP 321 QR code with their system scanner will be sent to YWallet instead of Zashi if both are installed on the device.