Title: header manipulation for DirectResponseAction

Description:
Our goal here is when we return an HTML file in a DirectResponseAction (i.e. a Forbidden page), to have it's content-type set to text/html. By default it's text/plain, rendering incorrectly in the browser. This could be done by setting the content-type response header but there is no way to specify response headers for just a DirectResponseAction that I can identify (proto struct). Another solution is to have Envoy infer the content-type for DirectResponseActions using the filesystem.

Currently, response headers can be modified at 1) the entire route configuration (doc), 2) the virtual host level (doc), or 3) in a single RouteAction (doc) which doesn't cover DirectResponseAction since they are "side-by-side".

Potential steps to resolve this are either

1. move the response_headers_to_add options from the RouteAction to the Route so that these options can apply regardless if the action is a RouteAction, DirectResponseAction, RedirectAction or any other future actions (preferred IMO but breaks the API)
2. Add response_headers_to_add options to DirectResponseAction, (and potentially) RedirectAction, and other future actions
3. Support the ability to detect content-type for DirectResponseAction based on filename extension if loading from file (easy but only solves a very narrow use-case, in this case ours)

This isn't a strict requirement we have to meet, but a feature that would be useful.

cc @rgs1 @brian-pane