Implement TPM2_VerifySignature

[dpsmith]

There are use cases where one may want to rely on a TPM to validate a signature made by another TPM.

[chrisfenner]

here are some other fun use cases:

• Use TPM to verify an HMAC or other symmetric signature while protecting the symmetric key
• Support TPM2_PolicyAuthorize (which requires you to first TPM2_VerifySignature the signed policy blob