validation: creating a validation listener manager (#24870)

This pulls the validation listener manager code into an extension, such that there's no hard-coded dependency on the TCP listener code. It should be a no-op for Envoy and a slight memory improvement for Envoy Mobile which does not support or use validation mode.

Risk Level: low
Testing: n/a
Docs Changes: n/a
Release Notes: n/a

Signed-off-by: Alyssa Wilk <alyssar@chromium.org>

Mirrored from https://github.com/envoyproxy/envoy @ ec9099786796da6f834a6d562d0c3939c342a5e1

envoy/config/listener/v3/listener.proto

@@ -379,6 +379,12 @@ message Listener {
 message ListenerManager {
 }
 
+// A placeholder proto so that users can explicitly configure the standard
+// Validation Listener Manager via the bootstrap's :ref:`listener_manager <envoy_v3_api_field_config.bootstrap.v3.Bootstrap.listener_manager>`.
+// [#not-implemented-hide:]
+message ValidationListenerManager {
+}
+
 // A placeholder proto so that users can explicitly configure the API
 // Listener Manager via the bootstrap's :ref:`listener_manager <envoy_v3_api_field_config.bootstrap.v3.Bootstrap.listener_manager>`.
 // [#not-implemented-hide:]