API changes for v0.6

[Bromeon]

Smaller API changes collected in one issue:

1. IAudioStream::instantiate_playback(&self) -> (&mut self)

   • Includes derived methods
   • See original comment

2. GDExtensionManager API should be mostly unsafe

   • See Panic when calling deferred reload through new typed API #1249 (comment)

3. We have AnyDictionary::cast_dictionary but no AnyArray::cast_array.
   Either add or remove both. Also needs ⚠️ sign.