Add metadata parameter to /dandisets/{dandiset_id}/ endpoint

[jwodder]

Making a request to /dandisets/$ID/?metadata=1 should return a response that includes "metadata" fields in the version objects containing the draft and lastest versions' metadata. This would cut down on some of the requests that dandidav has to make.

[yarikoptic]

We already have similar metadata flag for /dandisets/{versions__dandiset__pk}/versions/{versions__version}/assets/
https://github.com/dandi/dandi-archive/blob/HEAD/dandiapi/api/views/serializers.py#L373

❯ git grep -p 'metadata = serializers.Bool'
dandiapi/api/views/serializers.py=class AssetListSerializer(serializers.Serializer):
dandiapi/api/views/serializers.py:    metadata = serializers.BooleanField(required=False, default=False)

I think that pretty much every List serializer should gain such an option -- it would make use of the API more efficient, avoiding subsequent individual queries etc, and otherwise would come "for free" for us AFAIK.

[danlamanna]

Are there expected to be other fields in the future that could optionally be included? One pattern I've seen in other REST APIs is to have an include or expand parameter with a comma-delimited list of fields.

[jwodder]

@danlamanna I don't think version objects have any other fields to include.