Add an option to HttpClient's default DiagnosticHandler so that it does not create an Activity

Some of our developers are very surprised that DiagnosticHandler creates an Activity by default and they would appreciate a way that they can opt-out of this behavior. See [this thread](https://github.com/dotnet/runtime/issues/31862#issuecomment-677047256) for more background.
