Add support for using "app sessions" with the d.ecs identityprovider

[tlanfer]

This pull request implements support to use "app sessions" with the d.ecs identityprovider.

To use this in you app, you can either retrieve a full scim.Principal and as the sdk if its an app principal. This might be useful if your ressource can be called from both apps and real users.

principal = idp.PrincipalFromCtx(request.Context())
appName, isApp := principal.App()

if isApp {
	log.Infof(request.Context(), "Hello app %v", appName)
} else {
	log.Infof(request.Context(), "Hello user %v", principal.UserName)
}

Or, if your ressource is only ever to be used using an app principal, ask explicitly for that.

appName, isApp := idp.AppFromCtx(context.Background())

if isApp {
	log.Infof(request.Context(), "Hello App %v", appName)
} else {
	panic("illegal session") // bad idea
}