Remove blob_id from asset PUT/POST

ATM to update metadata blob_id is needed, it shouldn't. Even to create a new asset we do not really need blob_id as there must be etag digest in metadata record, dandi-api can identify the blob from the etag digest and thus completely avoid requiring blob_id in the API call.

While addressing this issue, please do not just remove blob_id from the end point but allow for some deprecation cycle so already released clients remain functionable. Just needs to verify that blob_id provided matches the one detected I guess

PS from a discussion with @satra last week, but I think we forgot to file this isue.

Start by locating the asset PUT/POST endpoints and the metadata validation that currently requires blob_id. Trace how the etag digest identifies a blob, then verify the compatibility behavior for clients that still provide blob_id. Done means new requests can omit blob_id, while supplied values are checked against the detected blob during the deprecation cycle.

Written by the indexing model from the issue text.