Transfer: Offline signature function.

[0xthreebody]

Need to solve first: #99

• Create wallet add offline mode (cold wallet), detect whether to open flight mode.
• Create a wallet to add observer mode, no private key storage, need to scan the QR code generated by the cold wallet, authorization.
• Cold wallet has get encoded data that needs to be signed to observe the wallet by scanning the qr code.
• Cold wallet have signature function (Sr25519 algorithm).
• Cold wallet has the function of generating signature information into qr code.
• Observe wallet has the function of scanning cold wallet signature qr code and broadcasting transactions.

Reference:https://github.com/maciejhirsz/uos