support shortcircuit in Request Modifiers

[minizilla]

Support shortcircuit in Request Modifiers by allowing to returns ResponseWrapper. Close #730, the issue doesn't have much comment but since this is trivial, I create a PR regardless.

[kpacha]

Hi @minizilla ,

Thanks for the contribution. I think that the feature makes a lot of sense but I don't think this solution is the optimal one. Instead of extending the semantics of the request modifier plugin, I'd create a new type of plugin (let's call it proxy plugin for now) that exposes a signature similar to the proxy.Proxy type/interface. With this implementation, we could solve 2 different requirements: being able to abort the pipe execution and having a shared context where the request and the related response are clearly available in a single function.

Ignoring all the boilerplate required for declaring the type and registering the factory, I think that proxy plugin could have a shape like this:

type ProxyWrapper func(context.Context, RequestWrapper) (ResponseWrapper, error)

func (registerer) registerProxyPlugin(globalCtx context.Context, cfg map[string]interface{}, next ProxyWrapper) ProxyWrapper {
	// check the config and, if the plugin shouldn't be injected, just return `next`
 	return func(ctx context.Context, r RequestWrapper) (ResponseWrapper, error){
	 	// plugin logic here. you could just return a response wrapper and an error in order to abort the pipe
	 	// or just delegate to `next` (maybe after altering the request) and use its results to generate what
	 	// the plugin will return
	 	return next(ctx, r)
 	}
}

This is a quick sketch and maybe the signatures will require some modifications (like using interface{} in order to avoid sharing types) but I think it's a good starting point for the discussion.