Support move to System.Text.Json on the Service Library

[andrueastman]

The service library currently uses the Newtonsoft.Json dependency from core. To have the service library move to System.Text.Json we need to achieve the following.

• Update the code generator to emit the proper attributes for System.Text.Json in the generated models
• Rewrite any model extensions here to support System.Text.Json
• Rewrite the ComplexTypeHelper.cs to support System.Text.Json
• Refactor any tests in Service Library with a Newtonsoft dependency
  AB#6305