[tuf] Use trusted_root.json to fetch CTLog/Rekor/

[asraa]

Description

@bdehamer recently added a trusted_root.json (https://github.com/sigstore/root-signing/blob/main/targets/trusted_root.json) that includes all trusted targets along with their metadata (like log ID).

The functions GetRoots(), GetIntermediates() and GetRekorPubs() should all be migrated to using this target file.

This does not require an API change!

Unfortunately, I cannot do this, but filing this issue because eventually clients should be relying on this target, and not in the individual files.

@znewman01 for sigstore-go tracking

cc @jku @haydentherapper

[haydentherapper]

We should also use the newly added expiration information and stop relying on the usage metadata for active vs expired.