fix: Callback signer

[tmathern]

For #124

• Fix callback signers bindings
• Builder.sign (and sign_file) return manifest bytes and status (it is quite useful for testing too, and for those who use the returned signed bytes)
• Clean up error message to move them up on class level so they're not recreated with each instance
• Add unit test: ⚠️ It seems the cryptography APIs have changed, so callback function is written accordingly ⚠️

For future reference: To get debug logs in-between languages: Log everything that can be logged - Python logger may be helpful to actually see the logs, combined with logging to stderr and flushing logs on a regular basis. Example in 8e791d1.