Include generated python bindings for protobufs

[chasen-bettinger]

In an effort to keep all of the different language implementations of in-toto consistent within the ITE-6 spec, a strategy we can use is to have each language client reference the Protobufs generated within this repository (Note: Go bindings have already started to take shape). For the Python client of in-toto, we can continue down this path laid by the Go bindings (impetus).

Goal:

Implement Protobuf Python bindings for the protobufs in the V1.0 spec.