set header for 'client' file for .well-known matrix homeserver discovery

I'd rather solve this with either <location> or <directory> but those are unavailable via .htaccess. Downside of this change is that any file named client will get matched and gets the Access-Control-Allow-Origin "*" header attached. This should be fine as long as we only have the one file that is supposed to be matched.

I tested whether I could add the path to the file in question in this directive, but that's unfortunately also unsupported.

If someone has a better solution, I'd like to implement that, but barring editing the apache conf file I don't see a simple solution other than this.