verifier: Fix TSA cert chain construction

[jku]

Using add_root_certificate() multiple times looks like a bug: it usually ends up working because

• the API accepts multiple roots and handles roots and intermediates in the same way
• typically the signing cert is actually included in the timestamp itself

However, in the case of timestamps with no embedded certs verification fails with

Certificates neither found in the answer or in the Verification Options

This is understandable since the rfc3161-client Verifier thinks it does not have the actual signing certificate (as it's actually added as root certificate). This case should succeed since our certificate chain contains the signing certificate -- so it is not required in the timestamp itself.

FYI @DarkaMaul

[jku]

This seems correct to me and fixes the conformance issue in sigstore/sigstore-conformance#230

[jku]

Added a regression test. Current main fails the test with

Error while verifying certificates: Unable to verify pkcs7 signature: ErrorStack([Error { code: 276824192, library: "PKCS7 routines", function: "PKCS7_get0_signers", reason: "signer certificate not found", file: "crypto/pkcs7/pk7_smime.c", line: 413 }])

[woodruffw]

Flagging, nonblocking: we could add these with add_root_certificate instead, since they're semantically root certs ("root" meaning "in the root program," not "is a self-signed certificate"). That would be slightly faster, since our verification would then be one-hop (TSR -> TSA -> TSA-CA) rather than having to chain TSA-CA up to its "root."

In practice that's probably a marginal performance benefit, however.

[woodruffw]

Thanks @jku!