Changes to static request handler

[sticilface]

Try number two:

As discussed in #999

Allow serving of static SPIFFS files with a cache-control header. Defaults to nothing, so existing sketches are not broken. Takes a const char * but is stored as string.

Also added the ability to serve gz files, and directories. The handler will serve the original file first if present, then the gzipped one. Directories can also be served statically with the same rules.

Examples

// serves all SPIFFS with 24hr max-age control
HTTP.serveStatic("", SPIFFS, "","max-age=86400"); 

// serves all of server directory with no-cache
HTTP.serveStatic("/server", SPIFFS, "/server", "no-cache");