Write a SpacetimeDB backend module in C# that defines a new sum type and uses it in a table.

TYPES
- Struct: Circle
  - Fields:
    - Radius: int
- Struct: Rectangle
  - Fields:
    - Width: int
    - Height: int
- Sum: Shape = Circle | Rectangle

TABLE
- Result
  - Struct: Result
  - Fields:
    - Id: int (primary key)
    - Value: Shape

REDUCERS
- SetCircle: given id:int and radius:int, insert exactly one row into Result
  - (Id=id, Value=Circle{Radius=radius})
