move verify_delegate() from Metadata to Signed

[jku]

verify_delegate() is currently an instance method of Metadata. It could be an instance method of Signed since it does not need anything from metadata and since this would lead to cleaner code.

• Generally code that uses tuf.repository can mostly look clean as it does not need to deal with the actual Metadata:

  root = repo.root()
  print (f"root version is {root.version}")
  

  This no longer works if one wants to verify a delegate just because of API design
• We could add a Signed.verify_delegate() and similar code should work:

  root = repo.root()
  root.verify_delegate("targets", metadata)
  

• nothing prevents leaving Metadata.verify_delegate() in place as a deprecated method

[jku]

I thought I had filed this already but failed to find it -- please close if I was dumb and an issue exists already

[lukpueh]

I thought I had filed this already but failed to find it -- please close if I was dumb and an issue exists already

We discussed this here: #2272 (comment), but I don't think we created an issue.

[jku]

I'm trying this and it looks like as a result I can also simplify Updater/TrustedMetadataSet a bit:

we currently have accessors like TrustedMetadataSet.root which are used like consistent_snapshot = trusted_set.root.signed.consistent_snapshot
However, verify_delegate() seems to be the only reason why these accessors are type Metadata, and not the Signed object.

[jku]

Just to document how this evolved:

This was the original proposal:

root = repo.root()
root.verify_delegate("targets", metadata)

that is now actually:

root = repo.root()
root.verify_delegate("targets", metadata.signed_bytes, metadata.signatures)

So API is a bit more complicated but Root is now not dependent on knowing about Metadata: this is great for future DSSE container format work linked above