frost-client: add message confirmation step to Comms trait

[conradoplg]

Closes #476

I also did a small cleanup of passing SendSigningPackageArgs around instead of just SigningPackage since the former will actually have all the information the user needs to confirm what they're signing.

We don't address it exactly as suggested; rationale:

Since the tool is not aware of the protocol using it it's not possible to do the verification itself. However, it's important to show the user what they're signing and ask them if they really want to sign the message. We introduce that along with a trait method similar to what was suggested. In the context of Zcash, this will be amended later to show the transaction plan of the transaction being signed. At that point we will likely introduce some Content-Type data that will allow the user to understand what kind of data they are signing.

I've edited #333 to reflect some of these details.

[natalieesk]

Looks good