Feature: Integrate TUF to Archivista to allow download attestations and policies using the TUF client (Witness)

Feature: Add support to add Policies to TUF
Requires: https://github.com/in-toto/archivista/milestones/1 completed

See this demo: https://www.youtube.com/watch?v=camLVqnVel8

Note: This milestone doesn't include Witness implementation