Get LinkML-derived Dandiset Pydantic model working with dandi-archive backend

[yarikoptic]

• check what backend does before it gives metadata record out through API e.g. to DLP and meditor
• if does not go through pydantic model (just spits out json from DB) make it go through Pydantic model we produce from linkml, and serialize back into json when giving out through API.
  • Verify that we do not loose metadata !

that will allow us to ensure that our metadata modified schema is tollerated by DLP and meditor ok