Public access to SignatureInfo fields

[chalford]

Currently, the SignatureInfo struct has no public fields, and the only method in its implementation is cert_chain(). As an SDK user, I'd like to understand the details of the signature - specifically getting access to the issuer field. This is a request to add more getter methods or public access modifiers on the fields.

A workaround currently exists by serializing the Manifest Store to JSON, then deserializing it back into an identical object structure in your own code.