Improve error handling in the API

[tomchop]

Additional validation mechanisms:

• Give pattern a minimum length, make location optional
• Discriminants
  • When receiving a request, this gives Pydantic a hint on how to differentiate object types when using typing Union. This means that validation errors will be only reported for a specific type.
  • Add a discriminant to entities in Requests
  • Add a discriminant to indicators in Requests
  • Add a discriminant to observables in Requests
  • Not necessary for DFIQ as we're creating objects differently.