Elsa 3.1 - csharp script - extend the scripting capabilities to include custom types

## Feature Request
Elsa 3.1 - csharp script - extend the scripting capabilities to include custom types

### Problem Overview
CSharp expression evaluator fails to execute - ((MyType)Output.LastResult).PropertyName with error
error CS0246: The type or namespace name 'MyType' could not be found (are you missing a using directive or an assembly reference?)

### Proposed Solution
Capability to add custom namespaces and assemblies to CSharpOptions