Dashboard Controls: Fix schema transformation for link placement (#114630)

* fix: add missing transformation for scenes -> save model v2

* fix: link placement transformation on the backend between schemas

* fix: update the openapi spec in the tests

* tes: add tests for `transformSceneToSaveModelSchemaV2`

* tests: extend conversion_test.go to cover link placements

pkg/tests/apis/openapi_snapshots/dashboard.grafana.app-v2alpha1.json

@@ -1877,6 +1877,10 @@
             "type": "boolean",
             "default": false
           },
+          "placement": {
+            "description": "Placement can be used to display the link somewhere else on the dashboard other than above the visualisations.",
+            "type": "string"
+          },
           "tags": {
             "description": "List of tags to limit the linked dashboards. If empty, all dashboards will be displayed. Only valid if the type is dashboards",
             "type": "array",