Alerting: introduce AlertQuery in definitions package (#63825)

* copy AlertQuery from ngmodels to the definition package
* replaces usages of ngmodels.AlertQuery in API models
* create a converter between models of AlertQuery
---------

Co-authored-by: Alex Moreno <alexander.moreno@grafana.com>

pkg/tests/api/alerting/api_backtesting_test.go

@@ -53,8 +53,7 @@ func TestBacktesting(t *testing.T) {
 	require.Truef(t, ok, "The data file does not contain a field `query`")
 
 	for _, query := range queryRequest.Data {
-		isExpr, _ := query.IsExpression()
-		if isExpr {
+		if query.DatasourceUID == "__expr__" {
 			continue
 		}
 		t.Logf("Creating a new test data source with UID %s", query.DatasourceUID)