Add route prefix for reverse proxy

[April-Q]

If Kubediag is behind a reverse proxy, particularly one where Kubediag is not at the root, this doesn't work so well.
For example, I have Nginx installed with the following configuration:

http {
 server {
   listen 0.0.0.0:30066;
   location /kubediag/ {
     proxy_pass http://localhost:9090/kubediag/;
   }
 }
}
events {
}

This configuration would serve Kubediag up on http://localhost:30066/kubediag/, and Nginx will include the /kubediag/ prefix when passing on requests to Kubediag. To make this work we should enable route prefix feature which will allow to control this more granularly:

kubediag-dashboard --web.route-prefix=/kubediag