ext_proc: Implement response path for headers only (#14713)

Implement header processing on the response path by sending the
response_headers message to the processor and handling the result.

Also update the docs in the .proto file.

Signed-off-by: Gregory Brail <gregbrail@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 9753819331d1547c4b8294546a6461a3777958f5

envoy/extensions/filters/http/ext_proc/v3alpha/ext_proc.proto

@@ -23,16 +23,16 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 // The External Processing filter allows an external service to act on HTTP traffic in a flexible way.
 
 // **Current Implementation Status:**
-// At this time, the filter will send a "request_headers" message to the server when the
-// filter is invoked from the downstream, and apply any header mutations returned by the
-// server. No other part of the protocol is implemented yet.
+// At this time, the filter will send the "request_headers" and "response_headers" messages
+// to the server when the filter is invoked, and apply any header mutations returned by the
+// server, and respond to "immediate_response" messages. No other parts of the protocol are implemented yet.
 
 // As designed, the filter supports up to six different processing steps, which are in the
 // process of being implemented:
 // * Request headers: IMPLEMENTED
 // * Request body: NOT IMPLEMENTED
 // * Request trailers: NOT IMPLEMENTED
-// * Response headers: NOT IMPLEMENTED
+// * Response headers: IMPLEMENTED
 // * Response body: NOT IMPLEMENTED
 // * Response trailers: NOT IMPLEMENTED
 
@@ -78,7 +78,6 @@ message ExternalProcessor {
   // The filter supports both the "Envoy" and "Google" gRPC clients.
   config.core.v3.GrpcService grpc_service = 1;
 
-  // [#not-implemented-hide:]
   // By default, if the gRPC stream cannot be established, or if it is closed
   // prematurely with an error, the filter will fail. Specifically, if the
   // response headers have not yet been delivered, then it will return a 500