Authenticate against Grafeas with PKI

When using Grafeas with Kritis one must setup certificates for the two services to communicate. This prevents Voucher from connecting to Grafeas as it presently doesn't support certificate auth against Grafeas OS.

To resolve this, we should add certificate support to the Grafeas MetadataClient:

• Load certificate from filesystem in cmd/config and pass it to the Grafeas MetadataClient.
• Load private key from filesystem (or ejson for consistency with PGP code?)