Provide option to read intermediates from timestamp struct

[haydentherapper]

Description

Intermediates can be distributed in the pkcs7 struct alongside the signing certificate. Currently we require that both roots and intermediates are provided to verify a signed timestamp - https://github.com/sigstore/timestamp-authority/blob/main/pkg/verification/verify.go#L247-L288

For BYO PKI use cases, for those that do not want to distribute intermediates as trusted material, we can support this by calling VerifyWithChain (code) or by populating the intermediate pool if no intermediate pool is provided.