What would you like to be added:

Add support for FULL_DUPLEX_STREAMED mode.

See https://github.com/envoyproxy/envoy/blob/60aed1c0ae520ed4e1d828c998ae961b4314542c/api/envoy/extensions/filters/http/ext_proc/v3/processing_mode.proto#L82-L111.

Why is this needed:

FULL_DUPLEX_STREAMED ensures the extension is able to receive arbitrarily large request body payload and defer a header response until the parsing is completed.