ext_proc: Support per-route GrpcService configuration. (#20757)

This adds the ability to change the GrpcService used by the ext_proc filter on a per-route basis.

Risk Level: Low. Not triggered unless configured.
Testing: New unit and integration tests added.
Docs Changes: Addition of new config field.

Signed-off-by: Michael Warres <mpw@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 0e8899c90213b39d8f4a1a083d4fd31e9c2fe8c1

envoy/extensions/filters/http/ext_proc/v3/ext_proc.proto

@@ -178,6 +178,7 @@ message ExtProcPerRoute {
 }
 
 // Overrides that may be set on a per-route basis
+// [#next-free-field: 6]
 message ExtProcOverrides {
   // Set a different processing mode for this route than the default.
   ProcessingMode processing_mode = 1;
@@ -195,4 +196,7 @@ message ExtProcOverrides {
   // Set different optional properties than the default setting of the
   // ``response_attributes`` field.
   repeated string response_attributes = 4;
+
+  // Set a different gRPC service for this route than the default.
+  config.core.v3.GrpcService grpc_service = 5;
 }