Add a new client_certificate criterion that accepts a "Certificate
Matcher" object. Start with two certificate match conditions:
fingerprint and SPKI hash, each of which can accept either a single
string or an array of strings.
Add new "client-certificate-ok" and "client-certificate-unauthorized"
reason strings.
Add a new reason "client-certificate-required" that will be returned by
the invalid_client_certificate criterion in the case that no client
certificate was provided. Determine this using the new 'presented' field
populated from the Envoy metadata.
