Create an endpoint to trigger asset summary generation

Yarik wrote:
> @dchiquito do you see an easy quick way to trigger that offband generation of summaries for all dandisets?
> may be we should expose some `/updateInternal` endpoint for each dandiset to facilitate such/similar updates in the future?

_Originally posted by @yarikoptic in https://github.com/dandi/dandi-api/issues/389#issuecomment-873415133_

I would recommend the endpoint `PUT /dandisets/{versions__dandiset__pk}/versions/{versions__version}/assets/summary` for this purpose.

Design doc: https://github.com/dandi/dandi-api/blob/master/doc/design/scheduled-tasks.md

Start by reading doc/design/scheduled-tasks.md and the issue's proposed PUT /dandisets/{versions__dandiset__pk}/versions/{versions__version}/assets/summary entry point. Done means this endpoint can trigger asset summary generation for the specified dandiset version, as described in the issue.

Written by the indexing model from the issue text.