Feat: toString() for queries

[Meldiron]

In Appwrite, we currently test queries by passing string versions into HTTP requests, such as

'queries' => [ 'limit(1)' ]

Thanks to introduction of toString(), we can now build actual query classes in Appwrite tests, such as

'queries' => [ Query::limit(1)->toString() ]

That will improve code quality and will make sure no unexpected test breaks occur when internal change is done to queries syntax.