Remove IApplicationEnvironment (v1 and v2) and just use abstract class in the SDK

[mslukebo]

Because the SDK targets netstandard2.0, we cannot make use of default interface implementations of methods. This causes headaches when we want to add new functionality to the application environment, because adding a member to the interface is a major breaking change.

If we get rid of the interfaces and just use the abstract class, we can use virtual members to add new features while maintaining backwards compatibility.