Add `PATCH` endpoints for assets

`PUT` endpoints to replace an entire document are already available, but typically users just want to update individual fields, for which a `PATCH` endpoint is normally used. We can rely on largely the same schema, but make each required attribute optional and figure out which fileds are set through `__fields_set__` (https://roman.pt/posts/handling-unset-values-in-fastapi-with-pydantic/). Branch `update/no-name` has some references on how to make fields optional programmatically. The `PUT` should support schema conversions between different versions of the REST API.