Add archivist support

[colek42]

*This issue requires upstream DSSE envelope support in in-toto

• The configuration should have a flag to upload the attestation to Archivist's public instance
• The configuration should allow the user to specify a private instance of Archivist.

[colek42]

https://github.com/testifysec/archivist

[adityasaky]

Relevant for DSSE support:
in-toto/in-toto-golang#148
in-toto/in-toto-golang#151