Report item schemas in API and use them to dynamically create UIs

[ml-evs]

General idea: we want to dynamically report the item types that a datalab supports. These might be defined outside of this repository at the deployment level, e.g., a datalab instance defines a new custom field on top of our Sample type. The server should then report the JSONSchema for the Sample type at runtime on the server.

This will involve 3 main steps:

• Add an API endpoints for reporting schemas #940
• Use dynamic schema info in UI #952
• Then actually extend our pydantic models with ways to hint about desired UI. This ties in with custom types needing custom components, e.g., date time. Add UI hints metadata system to Pydantic models Add UI hints metadata system to Pydantic models #1401
  
• Document all this!

Points that come to mind:

• determining/reporting type hierarchy
• versioning of schemas
• generally need to think about how we support custom schemas in datalab, of which this is a important part but not the whole story
• this approach, if successful, could also then be used for blocks