/search: be able to get those matching assets

[yarikoptic]

Brought up by @satra during meeting but I failed to find an issue.

ATM new /search returns list of dandisets with the number of assets matching the criteria. But it is impossible for a user to visualize/download those assets:

[satra]

it was on our internal slack, but here it is:

unless i missed it somewhere, it would be good to include a manifest download (a json array of assets) for each result and for the overall results (this could be generated on the fly when requested). i think it would be fine to start with a list of dandiset specific asset endpoints instead of the full metadata record per asset, but that could be an optional enhancement later. without such a mechanism individuals would have to do a search again through some other mechanism to actually get at which files were relevant.