ext_proc: Add support for processing_mode and processing mode override (#14822)

This lets the configuration control which calls are made to
the external processor, and also lets the processor temporarily
change the mode for a single request.

Risk Level: Low
Testing: unit test and integration tests, with new tests added to this PR to support the feature
Docs Changes: Removed "not-implemented-hide" from the "processing_mode" configuration parameter

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

Mirrored from https://github.com/envoyproxy/envoy @ 9976ba92afb3d2153887365260f6704b32ba9499

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

@@ -87,7 +87,6 @@ message ExternalProcessor {
   // or could not be opened, processing continues without error.
   bool failure_mode_allow = 2;
 
-  // [#not-implemented-hide:]
   // Specifies default options for how HTTP headers, trailers, and bodies are
   // sent. See ProcessingMode for details.
   ProcessingMode processing_mode = 3;