expose metadata from storage

[stevenctl]

Is your feature request related to a problem? Please describe.

Storage objects can have associated metadata, this was solved in supabase/storage#439. However, there is nothing in the dart/flutter API that exposes this metadata.

Describe the solution you'd like

Download would need to have a new response type or another method would need to be added:

// old
Future<Uint8List> download(String path, {TransformOptions? transform})

// new method
Future<StorageResponse> downloadWithMetadata(String path, {TransformOptions? transform});

class StorageResponse {
  Uint8List data;
  Map<string, dynamic> metadata; 
}

Similarly, upload needs additional parameters (or new fields in FileOptions?) to allow writing to metadata

Future<String> upload(
  String path,
  File file, {
  FileOptions fileOptions = const FileOptions(),
  int? retryAttempts,
  StorageRetryController? retryController,
})

Describe alternatives you've considered

Managing my own metadata separately from the storage objects. I'm not a fan of this; the storage schema has a metadata column per object.

Additional context

https://supabase.com/docs/guides/storage/schema/design

This is implemented in the javascript and python libraries afaict:

supabase/storage-py#328

supabase/storage-js#207

[stevenctl]

#1023

I see this was a dupe of #1023 and #1021