Importing and signing with multiple private keys

[Aaron Cox]

To start with we need to test the impact of modifying the WalletPluginPrivateKey to accept an array of private keys instead of just one. It would iterate over each key imported, signing the incoming transaction, and returning an array of signatures instead of just one.

This will be useful in situations where you have multiple authorities on the same action, and need to sign for each.

If the impact is too great or causes some unneeded side effects (when compared to the other wallet plugins), then perhaps we need to create and publish a WalletPluginMultiplePrivateKey wallet plugin that accomplishes this same behavior. Then, when developers need this specific kind of signing with multiple keys, they can just use this plugin instead of the WalletPluginPrivateKey that comes included with the Session Kit.