Draft: TUF find new, v2

[RebeccaMahany]

Relates to #954.

This is the next step toward using our new TUF infrastructure: a new implementation of findNew that looks at our new update library.

Future work + linked PRs

Subsequent PRs will tackle the following (order is not set in stone):

1. Point to release file <binary>/<os>/<arch>/<channel>/release.json, once available
2. Trigger a reload/restart after a successful update; launcher init performs version selection using the new update library (potentially falling back to findNew); launcher performs library tidying after version selection
3. Other improvements/refactors (ship a more up-to-date TUF repo, maybe split library parts of autoupdate/tuf out into its own package)
4. An "update now" functionality tied to control server
5. Eventually removing the old notary autoupdater

Previous work:

1. Run new TUF autoupdater side-by-side with notary autoupdater #1081
2. Expose data and metrics about new TUF autoupdater #1103
3. Point to production TUF infra #1108
4. Perform retry on TUF update #1110
5. Add library manager to handle TUF downloads #1111
6. Small autoupdate improvements #1119
7. Pass knapsack in to tuf autoupdater to simplify configuration #1168
8. [TUF autoupdater] Remove osquery client dependency #1178

[RebeccaMahany]

Closing in favor of #1185