Make non virtual properties virtual to enable mocking

[jabba2324]

When using a mocking framework like moq the properties of some request & reponse models cannot be overridden.

For example:

_loginResult = new Mock<LoginResult>();
_loginResult.SetupGet(r => r.AccessToken).Returns("my token");

Throws:
System.NotSupportedException : Invalid setup on a non-virtual (overridable in VB) member: r => r.AccessToken

Making these properties virtual should enable this with no side effects.

[leastprivilege]

I think I prefer that over the interface

[jabba2324]

@leastprivilege ok let me add the client class to this PR, done.

[leastprivilege]

OK - thanks!