Cache Control: Conditional Request and 304 Response Support

[ChihweiLHBird]

A previous PR provides cache-control HTTP header to file function in Sanic, but the best practice is to also implement the etag and the file validator.

Correct me if I am wrong:
With cache control value of no-cache, a validation is required every time before the client using the cached value. As my observation, validation request is usually a conditional request (requests that contain an If-None-Match or an If-Modified-Since header) to the resource URL. If the cached content is validated and the HTTP method is GET or HEAD, a 304 Not Modified response can be sent to the client, which is faster and smaller because it doesn't have to contain the file in the body. Validation is checking the conditions in a conditional request.

I think Sanic already supports 304 response in static file. I am wondering if we can do the same for file response in a general route handler.