Support ext_proc FULL_DUPLEX_STREAMED mode

[AndresGuedez]

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.

[ahg-g]

/assign @liu-cong

[ahg-g]

/assign @kfswain

[kfswain]

Let's keep this assigned to me, but I'm going to hold on this for a short time. We have some other folks working on this, so some of the logic may be reusable.

I have a PR out to EGW to see if we can make configuring this a little easier (i.e. just use the EnvoyExtensionPolicy rather than having to patch the xDS config directly)

[kfswain]

Update: I am implementing this, and we will reuse some of the logic for this implementation in #321