Sign transaction with the BTCPay App on BTCPay Server

[pavlenex]

Once we iron out other important details are cleared out, I would like to open a discussion about adding possibility for server users to sign transactions with the BTCPay App. I believe it can be a good security consideration and great balance between security and usability, and potentially a good UX when a Server User wants to use external on-chain wallet.

Scenario:

1. User generates an on-chain and LN wallet on the app itself, keys are on the app.
2. User wants to use server to send the funds out
3. For this user, we generate a watch only on-chain wallet on the server.
4. Ideal UX would be some sort of 2FA style signing

Lower priority for now, but kinda related to the UX #117 which might be unexpected error for the app itself, but is absolutely expected if app user went to the server and tried to send funds out, since it's a watch-only on-chain wallet on the Server.