Authz: Refactor folder tree (#99554)

* Refactor folder tree to its own structure

* Make it possible to json encode the tree

* Use iterations for Ancestors and Children

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Co-authored-by: IevaVasiljeva <ieva.vasiljeva@grafana.com>

pkg/services/authz/rbac/models.go

@@ -23,9 +23,3 @@ type ListRequest struct {
 	Verb         string
 	Action       string
 }
-
-type FolderNode struct {
-	UID          string
-	ParentUID    *string
-	ChildrenUIDs []string
-}