[Meta] Support for Timestamp Protocol verification

[DarkaMaul]

Improve the support of sigstore-python for signing and verification APIs and CLIs for accepting signed time from a TSA versus an artifact transparency log (or both).

Description

We developed a new rfc3161-client to perform the parsing and generation of the Timestamp Request/Response objects.

This set of changes aims to integrate this new client into sigstore-python. We'll split the contributions into several PRs to make their review slightly easier.

• Write the TimestampVerificationData models
• Update TrustedRoot to provide the Timestamps Authority information.
  ⚠️ This field is planned for removal ( [targets v11] What to do with the GitHub TSA in trusted_root.json root-signing#1268 , Signing event: sign/update-targets-1 root-signing-staging#156 )
• Add the Timestamp verification mechanism itself.
• Add a CLI parameter require-timestamp to trigger the verification

/cc @woodruffw @facutuesca

[jku]

Can I get a short explanation for the use cases:

Support for Timestamp Protocol verification

vs

This field is planned for removal ( [targets v11] What to do with the GitHub TSA in trusted_root.json root-signing#1268 , Signing event: sign/update-targets-1 root-signing-staging#156 )

If the public good instance is not going to support this, is it worth the additional code and dependency in this client?