cosign/tuf: use trustroot

[jleightcap]

Summary

Towards #280, adapt Cosign and TUF to use trustroot.

As a roadmap, this PR is structured:

• define the TUF trustroot types in src/tuf/repository_helper.rs.
• refactor the TUF implementation to use trustroot and rustls-webpki+rustls-pki-types.
  • note: this now means that the trustroot is "prefetch"ed, then used in async contexts--see release notes.
  • this also introduces some lifetime information that propagates into other components.
• misc. cleanup in changed components
  • pull the root JSON out of raw strings and into their own committed files.
  • Repository construction now uses the trustroot, with a FakeRepository option for BYO keys/certs--see release notes.

Release Note

The method of constructing a Repository with out-of-band trust materials (from Rekor public key and Fulcio certificate) has changed. See Cosign's verify example: https://github.com/sigstore/sigstore-rs/blob/a536beaa6c393e85e07f1d54cf01a4c7b2ec0258/examples/cosign/verify/main.rs#L231-L265

And the signature verification documentation: https://github.com/sigstore/sigstore-rs/blob/8a269a304c3b3ed1c3a6c89e4a6a88f9af22598a/src/lib.rs#L83-L103

Documentation

[jleightcap]

@woodruffw and @tnytown mentioned some ownership design considerations that I'm not sure I fully understood. A closer look at some lifetime choices e.g. https://github.com/sigstore/sigstore-rs/blob/559673c16ec13ecb27f1b067b0f4fffad675c8a4/src/cosign/client_builder.rs#L55-L62 would be fantastic.

[woodruffw]

@woodruffw and @tnytown mentioned some ownership design considerations that I'm not sure I fully understood. A closer look at some lifetime choices e.g.

@tnytown can confirm, but I believe the question was whether it makes sense to allow the interior 'a lifetime on CertificateDer<'a> to "cascade" throughout the refactor. The alternative would be to create an OwnedCertificateDer wrapper that maintains a self-reference, using a crate like self_cell.

This is exactly what PyCA Cryptography does, turning a Certificate<'a> into an OwnedCertificate:

https://github.com/pyca/cryptography/blob/4c07d8eb289aaa0fbcdbcf370724122905fade02/src/rust/src/x509/certificate.rs#L29C1-L36

Whether or not this actually makes sense to do, however, is ultimately a design question. If the sigstore-rs maintainers are okay with the lifetime, then it probably doesn't make sense to do 🙂