Add --export feature to link command

[fboucquez]

Atm, the link command creates and announces signed transactions to the network. The user may or may not need to enter private keys depending on the main account, operator account, and their cosigners if any.

This feature request is to extend the link command with an --export option that writes down into a hex string file or a QR the transactions to be signed and announced. Then the user could import this string/QR in an offline wallet to sign if the user is super strict about the security of the signing accounts. The link command would still be online to create the required transactions, but the signing and announcement would be done elsewhere.

Note that the link command doesn't necessarily need to be executed on the node's vpc. If the node admin has a copy of the target folder, for example, used to create the initial configuration, the current link command could be executed on the admin's personal computer.