HTTP: add ability to skip future callbacks

[mpwarres]

(This is somewhat related to #63, not sure if it's better to fold into that or keep as a separate issue)

ext_proc filter supports the notion of a processing mode override, in which an ext_proc ProcessingResponse for e.g. request headers can say: "skip future callbacks for this request", avoiding the cost of RPCs for request/response body.

For Proxy-Wasm, we might consider adding something analogous, for example a new proxy_action_t value indicating remaining non-lifecycle callbacks can be skipped, or allowing proxy_done to be called outside of proxy_on_done.

[PiotrSikora]

I was mentally bundling this with #63 (see: Done), but it doesn't hurt to have an explicit issue for that. Thanks!