Nested Folders: Fix /api/folders pagination (#79447)

* Nested Folders: Fix /api/folders pagination

We used to check access to the root folders after fetching them from the DB with pagination.
This fix splits logic for fetching folders in:
- fetching subfolders
- fetching root folders
and refactors the query for the latter so that is filters by folders with permissions

* Add tests

* Update benchmarks

pkg/services/folder/model.go

@@ -160,8 +160,8 @@ type GetParentsQuery struct {
 // return a list of child folders of the given folder.
 
 type GetChildrenQuery struct {
-	UID   string `xorm:"uid"`
-	OrgID int64  `xorm:"org_id"`
+	UID   string
+	OrgID int64
 	Depth int64
 
 	// Pagination options
@@ -169,6 +169,9 @@ type GetChildrenQuery struct {
 	Page  int64
 
 	SignedInUser identity.Requester `json:"-"`
+
+	// array of folder uids to filter by
+	FolderUIDs []string `json:"-"`
 }
 
 type HasEditPermissionInFoldersQuery struct {