Upgrade to go-tuf/v2

[RebeccaMahany]

The current version of go-tuf is go-tuf/v2, and it has departed significantly from the version of go-tuf we use. We would like to update to use go-tuf/v2 to be able to pull in security patches and new features.

Upgrading to this new version of the package is nontrivial. I put together a first pass at some changes in this PR to get a sense of the scope.

We likely want to wait for theupdateframework/go-tuf#593 to be completed before upgrading to v2. However, we can get started ahead of time by replacing our direct usage of go-tuf with interfaces that we can swap out more easily later.

• Replace direct usage of go-tuf with interface in ee/tuf, TUF checkup, TUF info table, and packaging code
• Update to use go-tuf/v2
• Extensive testing