Make proxy_ directives configurable per Product

The request is to implement a Policy that enables to configure the proxy_* directives such as proxy_buffering, proxy_temp_path, and proxy_cache_path individually for each Product.

The use case here is for providing more control over how requests are written to disk (temporarily) for security purposes.

Requested tunable directives:

proxy_request_buffering
proxy_buffering
proxy_buffers
proxy_buffer_size
proxy_busy_buffers_size
proxy_temp_path
proxy_cache_path

The `proxy_request_buffering` directive is also required to transmit `Transfer-Encoding: chunked` requests without modification to the upstream.

Ideally this feature would be implemented similar to the Upstream Connection timeout policy in which the fact that it controls NGINX directives is irrelevant. And would implement the necessary code to make sure that their function worked with forward proxy configurations (such as when used in combination with the Camel Service Policy).