Fix C# Tags for Enums

[jdetter]

Description of Changes

• In C# we need to tag enum members with a [SpacetimeDB.Enum] tag, otherwise our custom serialization logic for enums won't work. This either has been broken or has never worked. If you have some types in rust like this:

#[SpacetimeType]
pub enum EnumValue {
  Item,
  Cargo
}

#[SpacetimeType]
pub struct MyType {
 pub value: EnumValue
}

These types need to be generated in C# as:

public enum EnumValue {
  Item,
  Cargo
}

public class MyType {
    [SpacetimeDB.Enum]
    public EnumValue value;
}

Before this PR, the tag was being omitted.

API

• This is a breaking change to the module API
• This is a breaking change to the ClientAPI

If the API is breaking, please state below what will break

[dbrinkmanncw]

Tested and working