Add the ability to verify signed bundle against trusted root

[steiza]

Summary

As we work towards releasing signing support (#136), we want the option to verify that the bundle we signed is valid before returning it to the user.

At first I thought about requiring a trusted root, but I came up with at least two examples where that wasn't feasible:

1. Staging doesn't include a timestamp authority that is reachable from the general internet (arguably this could be fixed)
2. Conformance tests that use detached materials would require a custom verification policy, at which point you might as well use verify directly

Release Note

NONE - we aren't ready to release signing yet

Documentation

N/A

[haydentherapper]

Staging doesn't include a timestamp authority that is reachable from the general internet (arguably this could be fixed)

I didn't catch that we were using the sigstore/timestamp-authority client in the signer. This limitation of this client is that the API of the TSA must match the one defined in Sigstore's implementation which is not standardized. The only standardization is what the API must accept and produce. This is why in Cosign, we implemented a client where you must pass the API path rather than just the root URL, which means that we can support any TSA. We can upstream the Cosign TSA client to sigstore/timestamp-authority as well.

If we do this, then you can use https://freetsa.org/index_en.php for staging.

Edit: Concretely, all that needs to change is that when you specify BaseURL when initializing the TSA client, you would just provide the entire API, e.g. https://timestamp.githubapp.com/api/v1/timestamp, which is a reasonable tradeoff to support all TSAs imo.

Edit edit: I also realized that doesn't exactly answer your question, so in addition to this change, we'd need to append the custom timestamp authority to the trust material.

Edit edit edit: I keep finding more things :) In the signing client config, we also define a TSA URL as its whole path - https://github.com/sigstore/protobuf-specs/blob/main/protos/sigstore_trustroot.proto#L176-L178. I would suggest we make this change (in another PR) to align here and with the other clients.

[kommendorkapten]

LGTM