PluginInterface

Base plugin interface. There is a lot of "duck programming" happening in this API that will go away once we have better type variance in PHP 7.4: https://wiki.php.net/rfc/covariant-returns-and-contravariant-parameters

Ideally, this interface would provide apply(SchemaComponent) and implementations could do apply(ModelQuery) using type variance, meaning we could get rid of virtually all PluginInterface descendants.