Support crypto-request and crypto-response UR flow

[ghost]

https://github.com/BlockchainCommons/Research/blob/master/papers/bcr-2021-001-request.md
https://github.com/BlockchainCommons/crypto-commons/blob/master/Docs/crypto-request-or-crypto-psbt.md

Implementing a request-response pattern like this would potentially offer a way to accomplish "headless signing" mentioned here:
#151

In a headless mode of operation, a wallet vendor could output a crypto-request that would be saved to a microSD card that Krux would read and process, then output its own crypto-response file back to the card for the wallet software to read. It would offer a standard way to solve the process of exporting an xpub and signing a PSBT without (direct) user intervention.

In the normal GUI mode of operation, implementing this flow wouldn't do much to benefit Krux other than potentially down the line allowing us to consolidate the Scan Address and Sign PSBT options into a more general Scan Request option (if we want that; I'm skeptical of the utility). The first screen to load or generate mnemonics would stay the same, and users would likely still want to see and print their Mnemonic, Extended Public Key (and key origin info), etc. That being said, the ability to handle these new UR types in the existing Sign PSBT method will probably become more important as time goes on.