Make Basic Auth realm filterable

[goldenapples]

I've been running into some frustration using basic auth on multisite installations because each network site uses a different Realm value and has to be authenticated to separately.

https://github.com/humanmade/PHPBasicAuth/blob/master/inc/namespace.php#L146

I think that it's possible for different domains to share basic auth state if they use the same realm. If this is possible, it would be nice to either (a) use a single authentication realm for a network by default, or (b) wrap the realm value in an apply_filters call, and allow sites to define their own logic for enabling authentication. (or both)

If this is not possible, then this issue can be closed as invalid.

Relevant documentation: https://tools.ietf.org/html/rfc2617

[goldenapples]

Did a bit of digging, and it looks as though it is not possible to share Basic Auth credentials between multiple domains (even subdomains of a shared root domain).

Browsers distinguish areas that require HTTP authentication by a combination of the URL root and the name of the authentication realm.

So even if the value served for "Realm" is the same between different sites on a network, the user will still be prompted separately for authentication on each site if the domains don't match.

An alternative which would work is HTTP Digest Auth (see section 3 of http://www.faqs.org/rfcs/rfc2617.html for details). Sample code for parsing digest authentication in PHP is available on the PHP manual page: https://www.php.net/manual/en/features.http-auth.php