Support nanoblock: QR

[ghost]

https://docs.nano.org/integration-guides/the-basics/#process-a-json-blob-block

If Natrium can broadcast a block to the network, transactions can be created on air gapped devices (cold storage computers, disconnected smartphones, and so on) which will generate a QR with the signed block.

Natrium can then be used to conveniently broadcast the block without ever having to connect the cold storage device to a network.