StaticFileMiddleware serve pre-compressed file when found by file provider

[msschl]

Is your feature request related to a problem? Please describe.

Today, most sites have a build system in place to build, uglify, compress, remove unused code from static web assets such as css, js, or other assets. One such a build system could be webpack as an example. Furthermore, some sites might use such build systems to pre-compress some static assets to reduce the runtime load. The StaticFileMiddleware added by UseStaticFiles has currently no ability to serve a pre-compressed file if present or fallback to the uncompressed file.

Describe the solution you'd like

It would be nice to add an opt-in option to the StaticFileOptions to enable serving pre-compressed files if present or otherwise falling back to the original uncompressed file.

The implementation would be straight forward I think. In the StaticFileContext.cs#L115 the file provider could be additionally queried for a compressed version of the requested file according to the accept-encoding request header. If a version of a pre-compressed file exists the StaticFileContext would serve the pre-compressed file and the content-encoding would be set accordingly. The content-type would be unaffected as it is retrieved by IContentTypeProvider from the originally requested path.
If there is no pre-compressed file available (i.e. the IFileInfo.Exists property returned by the file provider is false), the StaticFileContext would fallback and serve the normal uncompressed file.

[davidfowl]

cc @DamianEdwards @javiercn

[Tratcher]

Duplicate of #2458