Grafeas Transport: Record type information for fields converted to string for the Grafeas in-toto format

[adityasaky]

Before pushing in-toto metadata to Grafeas server, we're performing some serialization changes in the fields (which can be reversed for final verification). We should record the data type for each field that is converted to a string (especially of note in environment and byproducts), perhaps as a field itself in the environment map.