AdalException needs to be serializable

[MarkZuber]

In CustomWebUI flows, it's possible that the process implementing the UI is in another process.
Want to add a few methods to AdalException to support serializing the exception(s) in this case

string ToJsonString() -- serializes the AdalException (or any derived exception) into a json string which can be marshaled across the process boundary.

static AdalException FromJsonString(string json) -- factory method to create the appropriate (possibly derived) AdalException from the serialized json string.

[MarkZuber]

#1539