validation - add support for vendorization record from the instance server

As a part of the vendorization effort we need to allow for using vendorization from the server, see
- https://github.com/dandi/dandi-schema/issues/299

So we need
- for `dandi validate`
- add `--instance` option like we have for other commands
- if provided, talk to that instance server (from our `known_instances`) and discover vendorization record
- apply vendorization record before importing `dandi-schema` so it is in effect
- for `dandi upload`
- after instance is figured out, consult server for the vendorization record, and perform similar vendorization of dandi-schema import

Related
- https://github.com/dandi/dandi-archive/pull/2386 would provide instance specific JSONSchema, but we would not get vendor specific dandischema.