SSE: Add functions that determine NodeType by UID and construct a data source struct from NodeType (#70106)

* add NodeTypeFromDatasourceUID and DataSourceModelFromNodeType()
* deprecate expr.DataSourceModel
* replace usages of IsDataSource to NodeTypeFromDatasourceUID 
* replace usages of DataSourceModel to DataSourceModelFromNodeType()

pkg/expr/service_test.go

@@ -62,7 +62,7 @@ func TestService(t *testing.T) {
 		},
 		{
 			RefID:      "B",
-			DataSource: DataSourceModel(),
+			DataSource: dataSourceModel(),
 			JSON:       json.RawMessage(`{ "datasource": { "uid": "__expr__", "type": "__expr__"}, "type": "math", "expression": "$A * 2" }`),
 		},
 	}
@@ -124,3 +124,8 @@ func (me *mockEndpoint) QueryData(ctx context.Context, req *backend.QueryDataReq
 	}
 	return resp, nil
 }
+
+func dataSourceModel() *datasources.DataSource {
+	d, _ := DataSourceModelFromNodeType(TypeCMDNode)
+	return d
+}