HTTP: add more control over buffering and/or offloading processing to the host

[PiotrSikora]

Currently, we only support Continue and Pause actions, which is quite limiting.

Most notably, we require forwarding HTTP headers before processing HTTP body, which prevents building plugins that perform authorization based on contents of HTTP request body or completely consume requests and generate responses from within plugin (for which proxy_send_local_response is too limited).

Refs:

• Always process HTTP headers before processing HTTP body. proxy-wasm-cpp-host#95
• OnRequestBody not called if onRequestHeaders returns stopIteration proxy-wasm-cpp-host#143
• Can we support FilterHeadersStatus::StopIteration ? proxy-wasm-cpp-host#367
• re support the StopIteration status proxy-wasm-cpp-host#417

[jcchavezs]

Another use case is to decide whether buffer or not based on headers e.g. in Coraza WAF we only want to buffer and inspect body if certain content type.