PanelLibrary: adding library panels to Dashboard Api (#30278)

* Wip: First naive impl

* Chore: fix after merge

* Chore: changes after PR comments

* Chore: removes unused types

* Chore: adds feature toggle

* Refactor: adds library panels cleanup and connect when storing dashboards

* Refactor: adds feature toggle

* Update pkg/services/librarypanels/librarypanels.go

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Update pkg/services/librarypanels/librarypanels.go

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Refactor: adds disconnect library panels when deleting a dashboard

* Chore: changes after PR comments

* Tests: adds tests for LoadLibraryPanelsForDashboard

* Tests: adds tests for CleanLibraryPanelsForDashboard

* Tests: adds tests for ConnectLibraryPanelsForDashboard

* Tests: adds tests for DisconnectLibraryPanelsForDashboard and small refactor

* Update pkg/services/librarypanels/librarypanels_test.go

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Update pkg/services/librarypanels/librarypanels_test.go

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Update pkg/services/librarypanels/librarypanels_test.go

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Update pkg/services/librarypanels/librarypanels_test.go

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

* Refactor: deletes all connections in one call and connects all in the same transaction

* Chore: adds better comments

* Chore: changes after PR comments

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>

pkg/components/simplejson/simplejson.go

@@ -181,6 +181,17 @@ func (j *Json) GetIndex(index int) *Json {
 	return &Json{nil}
 }
 
+// SetIndex modifies `Json` array by `index` and `value`
+// for `index` in its `array` representation
+func (j *Json) SetIndex(index int, val interface{}) {
+	a, err := j.Array()
+	if err == nil {
+		if len(a) > index {
+			a[index] = val
+		}
+	}
+}
+
 // CheckGet returns a pointer to a new `Json` object and
 // a `bool` identifying success or failure
 //