Enhancements to the in-toto Jenkins plugin

[adityasaky]

The in-toto plugin requires some enhancements. This issue is a starting point to collect and coordinate efforts to add them.

• Support for Grafeas as a transport (Add Grafeas transport #10): this is currently underway. This would allow users to push in-toto links generated in a Jenkins pipeline directly to a Grafeas instance which includes the in-toto metadata kind (Add in-toto attestations to Grafeas grafeas/grafeas#391).
• Adding support for Grafeas requires a translator to handle the slight differences between the in-toto document format and the Grafeas document format. Handled in Add Grafeas transport #10.
• Code cleanup: there was some discussion on Add Grafeas transport #10 about cleaning up the implementation of GenericCRUD though this belongs in a separate PR (Add Grafeas transport #10 (comment)).
• The in-toto plugin currently doesn't record the command that was run as part of the step, and instead leaves it blank, similar to the behavior of using --no-command with in-toto-run.
• Improving unit tests, for Grafeas especially.