Add nullable annotations to reference assembly

The custom step APIs in the linker's reference assembly should have nullable annotations. When I tried to enable them, ApiCompat produced errors about NullableContextAttribute that I'm not sure how to work around:
```
error : CannotRemoveAttribute : Attribute 'System.Runtime.CompilerServices.NullableContextAttribute' exists on 'Mono.Linker.MessageOrigin..ctor(Mono.Cecil.IMemberDefinition, System.Nullable)' in the contract but not the implementation. [/home/sven/linker/src/linker/Mono.Linker.csproj
```

I think this is happening because the compiler decides how to emit NullableContextAttribute based on which annotations are most common in a type, which could be different in the ref assembly.

For future reference, we also need to disable CS8597 to prevent warnings about "throw null" in the ref assembly.