[Feature Request] Make sure all client responses are user-instantiable/mockable

[cretz]

Describe the solution you'd like

Today some things, such as workflow list result, are only instantiable via internal constructor which makes it difficult for users to test. Try to make everything that could come back from a client able to be instantiated by a user. What many clients do (e.g. just checked Azure clients) is make their classes simple classes with parameterless constructors and have all properties also have setters. This may be preferred over our current stance of getter-only properties that cannot be customized in test situations.