[Fuzz] Network-layer filter generic fuzzer (#12086)

* added generic freamework for testing filters.
This is a fuzzer for testing network-layer(L3/L4) filters.
Now Envoy has 20 network-layer filters which will deal with raw bytes from untrusted networks and thus they are security-critical to some extent. The idea of this is to write a fuzzer which can be applied to different kinds of network filters(potentially cover all the filters), and when new filters are added to Envoy, we won't need to write dedicated fuzzers one by one to give them fuzz coverage.

Signed-off-by: jianwen <jianwendong@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ ce26fe19e0f8cd033e16aa3a9145281b0d03b748

envoy/extensions/filters/network/client_ssl_auth/v3/client_ssl_auth.proto

@@ -29,7 +29,8 @@ message ClientSSLAuth {
   // the authentication service. The filter will connect to the service every 60s to fetch the list
   // of principals. The service must support the expected :ref:`REST API
   // <config_network_filters_client_ssl_auth_rest_api>`.
-  string auth_api_cluster = 1 [(validate.rules).string = {min_bytes: 1}];
+  string auth_api_cluster = 1
+      [(validate.rules).string = {min_bytes: 1 well_known_regex: HTTP_HEADER_VALUE strict: false}];
 
   // The prefix to use when emitting :ref:`statistics
   // <config_network_filters_client_ssl_auth_stats>`.