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.
Move downstream mTLS settings to a nested config file object, under the
key 'downstream_mtls', and add a new DownstreamMTLSSettings struct for
these settings.
Deprecate the existing ClientCA and ClientCAFile fields in the Options
struct, but continue to honor them for now (log a warning if either is
populated).
Delete the ClientCRL and ClientCRLFile fields entirely (in current
releases these cannot be set without causing an Envoy error, so this
should not be a breaking change).
Update the Settings proto to mirror this nested structure.
