Alerting: Add stored screenshot utilities to the channels package. (#49470)

Adds three functions:
`withStoredImages` iterates over a list of models.Alerts, extracting a stored image's data from storage, if available, and executing a user-provided function.
`withStoredImage` does this for an image attached to a specific alert.
`openImage` finds and opens an image file on disk.

Moves `store.Image` to `models.Image`
Simplifies `channels.ImageStore` interface and updates notifiers that use it to use the simpler methods.
Updates all pkg/alert/notifier/channels to use withStoredImage routines.

pkg/services/ngalert/state/manager_private_test.go

@@ -21,9 +21,9 @@ type CountingImageService struct {
 	Called int
 }
 
-func (c *CountingImageService) NewImage(_ context.Context, _ *ngmodels.AlertRule) (*store.Image, error) {
+func (c *CountingImageService) NewImage(_ context.Context, _ *ngmodels.AlertRule) (*ngmodels.Image, error) {
 	c.Called += 1
-	return &store.Image{
+	return &ngmodels.Image{
 		Token: fmt.Sprint(rand.Int()),
 	}, nil
 }
@@ -40,7 +40,7 @@ func Test_maybeNewImage(t *testing.T) {
 			true,
 			&State{
 				State: eval.Alerting,
-				Image: &store.Image{
+				Image: &ngmodels.Image{
 					Token: "erase me",
 				},
 			},
@@ -60,7 +60,7 @@ func Test_maybeNewImage(t *testing.T) {
 			&State{
 				Resolved: true,
 				State:    eval.Normal,
-				Image: &store.Image{
+				Image: &ngmodels.Image{
 					Token: "abcd",
 				},
 			},
@@ -71,7 +71,7 @@ func Test_maybeNewImage(t *testing.T) {
 			false,
 			&State{
 				State: eval.Alerting,
-				Image: &store.Image{
+				Image: &ngmodels.Image{
 					Token: "already set",
 				},
 			},