This FileServer is adapted from the one in net/http by the Go authors.

ListenAndServeTLSWithSNI serves TLS with Server Name Indication (SNI) support, which allows multiple sites (different hostnames) to be served from the same address.

New creates a new Server which will bind to addr and serve the sites/hosts configured in configs.

Server represents an instance of a server, which serves static content at a particular address (host and port).