Implement ETag

[akubera]

ETags are a core feature of web servers that users should expect to work out of the box. This should be a non-middleware feature baked into either Application or HTTPRequest classes, with the option to disable or swap implementations if the user so chooses.

[nudzo]

Get inspired: caddyserver/caddy#191

[akubera]

Thanks for your suggestion, @nudzo. Do you think it's best to just integrate etags into the static middleware vs some other, more general, mechanism?

[nudzo]

Yes, one I mentioned above is for static content.
Dynamic content best when generate ETags itself, cause it can do it the most effective way - based on version of doc, or hash generated on doc save, or similar. Otherwise you have to calculate some kind of hash on every response - performance impact.