UserAuthProvider's AuthCodeClient will be better of a protected property

[JamesAtPropertyMe]

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We are using 4.5.0 version of this library because our project is using ServiceStack 4.5.0. UserAuthProvider's AuthCodeClient is using IdentityModel 1.11.0 but our project has used IdenittyModel 3.9.0 elsewhere. This kind of settings makes UserAuthProvider fails to call TokenClient and returns this error: Method not found: 'Void IdentityModel.Client.TokenClient..ctor(System.String, System.String, System.String, IdentityModel.Client.AuthenticationStyle)'

It'd be nice if UserAuthProvider's AuthCodeClient is a protected property so that we can easily just override it without overwriting the whole UserAuthProvider class. Otherwise, it'd also good if there is some override-able factory pattern to construct a UserAuthProvider.