Make sure that signing key generation returns the public component

[marsella]

When creating a signing key (either locally generated or generated at the server), the caller needs to get back the corresponding public key (which is subsequently used to verify signature, store in a PKI, etc.).

• [x] Update the spec to clarify this in all signing key generation methods
• [x] Also update spec to clarify this in import methods?
• [ ] Make a corresponding issue in key-mgmt to fix

[marsella]

The generate protocol is implemented in two combinations right now: • local generation for arbitrary secrets • remote generation for signing keys

The arbitrary secret implementation doesn't need any changes as a result of this change. The signing keys implementation was fixed to return a public key in boltlabs-inc/key-mgmt#308.

The import protocol is implemented in one combination: • remote import for signing keys

This does need to be updated to return the public key. EDIT: See boltlabs-inc/key-mgmt#324.