SDK resolvers should have an API to clear state

SDK resolvers have caches because they depend on certain things such as files on disk to determine paths to SDKs. In order to make the cache more effective, SDK resolvers can make these caches `static` so they can be re-used by evaluations within the same process, especially Visual Studio. However, a long lived process like `devenv.exe` can cause these caches to potentially grow. It would be good if MSBuild had an API that could tell SDK resolvers to clear their state. For example, in Visual Studio when you close a solution, the SDK resolvers' caches could be cleared.