Feature request: static file serving support

[matthiask]

From #97

Nginx serves static files

Now that can be (IMO) a more interesting feature request for Granian.

I think it would be very useful to have some sort of static file serving functionality.

Either something like nginx's try_files where granian could first check a folder for a match, and if not, fall back to the RSGI/ASGI/WSGI app, or maybe just support mapping URL paths to folders.

A plus would be if granian could optionally serve those files with far-future expiry headers. nginx does the following when using expires max:

The max parameter sets “Expires” to the value “Thu, 31 Dec 2037 23:55:55 GMT”, and “Cache-Control” to 10 years.

From https://nginx.org/en/docs/http/ngx_http_headers_module.html

This is the only feature which is missing for me to make granian a one-stop solution to serve everything with a single ingress, no sidecars or similar.

(I'd be willing to work on this with some guidance, or fund it if that's helpful.)

[gi0baro]

I think this would be a useful feature for a lot of deployments out there, I definitely agree this should be implemented in the near future. I don't think this will be in time for 1.7, but possibly we can target 1.8.

In terms of implementation, here are some notes I can think of at the moment:

• we would need 2 additional configuration parameters, one for the folder path to serve, one for the URL path to expose that folder to (might be smth like --static-path-mount my_app/static --static-patch-route /static)
• this potentially affects only the build_service! macro here https://github.com/emmett-framework/granian/blob/a6cfc2f1f9d94ae7f57d6400cfee071eac328aa5/src/workers.rs#L169-L180 thus the best implementation would probably be to add a new macro and switch which macro gets used by the accept loop based on wether a static path is defined (on worker boot)
• the main attention point should be on the security of that endpoint: the implementation should do some type of checks to avoid exploiting that path going outside of the target folder (eg: avoid /static/../../../some_file_in_root_path)
• the expiration can probably be an additional param, I don't see any particular difficulties there