Support intermediate certificates in VerificationMaterial certificate chain

[steiza]

Description

Currently, in https://github.com/sigstore/sigstore-go/blob/main/pkg/bundle/bundle.go when we get a bundle that includes a certificate chain in the verification materials, we assume there is only 1 leaf certificate in the "chain" and no intermediates.

This needs to change if we're going to support certificate chains that include intermediates.

This came up in #101 (comment).