Upload a file

Uploads a file to an existing bucket.

Policy permissions required:
- buckets permissions: none
- objects permissions: insert


final avatarFile = File('path/to/file');
final String path = await supabase.storage.from('avatars').upload(
      'public/avatar1.png',
      avatarFile,
      fileOptions: const FileOptions(cacheControl: '3600', upsert: false),
    );