Add Asset Metadata

- [ ] Documentation
- [ ] Missing a handful of metadata params

Usage:  Retrieve basic metadata around assets (title, notes, description, custom_id)

```ruby
content_owner = Yt::ContentOwner.new({})
asset = Yt::Collections::Assets.new(auth: content_owner).where(id: 'A969176766549462').first

asset.metadata_mine.title #=> "Master Final   Neu La Anh Fix"
asset.metadata_effective.title #=> "Neu la anh"
```

To reduce n+1 requests, you can specify the metadata on the collection level as well:

```ruby
asset = Yt::Collections::Assets.new(auth: content_owner).where(id: 'A1234,A2345,A3456', fetch_metadata: 'mine')
```
Not a huge fan of this recursion, but I couldn't see an easier way off the top of my head without bigger changes than I had time for now.