Add an "enforcement" option to the new downstream mTLS configuration
settings group.
When not set, or when set to "policy_default_deny", keep the current
behavior of adding an invalid_client_certificate rule to all policies.
When the enforcement mode is set to just "policy", remove the default
invalid_client_certificate rule that would be normally added.
When the enforcement mode is set to "reject_connection", configure the
Envoy listener with the require_client_certificate setting and remove
the ACCEPT_UNTRUSTED option.
Add a corresponding field to the Settings proto.
Update isValidClientCertificate() to also consult the configured
certificate revocation lists. Update existing test cases and add a new
unit test to exercise the revocation support. Restore the skipped
integration test case.
Generate new test certificates and CRLs using a new `go run`-able source
file.
