IDForwarding: Add auth hook to generate id token (#75555)

* AuthN: Move identity struct to its own file

* IDForwarding: Add IDToken property to usr and identity structs and add GetIDToken to requester interface

* Inject IDService into background services

* IDForwarding: Register post auth hook when feature toggle is enabled

pkg/services/auth/identity/requester.go

@@ -62,6 +62,9 @@ type Requester interface {
 	HasUniqueId() bool
 	// AuthenticatedBy returns the authentication method used to authenticate the entity.
 	GetAuthenticatedBy() string
+	// GetIDToken returns a signed token representing the identity that can be forwarded to plugins and external services.
+	// Will only be set when featuremgmt.FlagIdForwarding is enabled.
+	GetIDToken() string
 }
 
 // IntIdentifier converts a string identifier to an int64.