[extension inplace change the ImmediateResponse::body type from string to bytes (#33563)

* inplace change the ImmediateResponse::body type from string to bytes

per discussion this should be a safe swap for c++ Envoy

Signed-off-by: Xin Zhuang <stevenzzz@google.com>

* add a unit test for non-utf8 body in extenal immediate response

Signed-off-by: Xin Zhuang <stevenzzz@google.com>

---------

Signed-off-by: Xin Zhuang <stevenzzz@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ ab4b6f70b283905d5fcb5ddb0b0be34562873b51

envoy/service/ext_proc/v3/external_processor.proto

@@ -339,7 +339,7 @@ message ImmediateResponse {
 
   // The message body to return with the response which is sent using the
   // text/plain content type, or encoded in the grpc-message header.
-  string body = 3;
+  bytes body = 3;
 
   // If set, then include a gRPC status trailer.
   GrpcStatus grpc_status = 4;