php: Add enabledExtensions attribute to PHP derivations

This provides a means to build a PHP package based on a list of
extensions from another.

For example, to generate a package with all default extensions
enabled, except opcache, but with ImageMagick:

php.withExtensions (e:
  (lib.filter (e: e != php.extensions.opcache) php.enabledExtensions)
  ++ [ e.imagick ])