PortfolioMetricsUpdateTask - notification

### Current Behavior

After `PortfolioMetricsUpdateTask` happens, it doesn't seem that any notification gets triggered.

I've configured console notification for all portfolio notifications.

e.g. logs:

```
2023-03-09 09:35:51,461 INFO [PortfolioMetricsUpdateTask] Executing portfolio metrics update
2023-03-09 09:35:51,471 INFO [ProjectMetricsUpdateTask] Executing metrics update for project 80091b58-c82c-4d40-bd92-7d818ff58424
2023-03-09 09:35:51,472 INFO [ProjectMetricsUpdateTask] Executing metrics update for project 2cd77d61-38ff-479a-bfad-a8e59f38c366
2023-03-09 09:35:55,650 INFO [ProjectMetricsUpdateTask] Completed metrics update for project 80091b58-c82c-4d40-bd92-7d818ff58424 in 00:04:178
2023-03-09 09:35:56,343 INFO [ProjectMetricsUpdateTask] Completed metrics update for project 2cd77d61-38ff-479a-bfad-a8e59f38c366 in 00:04:871
2023-03-09 09:35:56,352 INFO [ProjectMetricsUpdateTask] Executing metrics update for project 36ffe57f-cc8d-489f-b64f-2f9a083a1096
2023-03-09 09:35:56,352 INFO [ProjectMetricsUpdateTask] Executing metrics update for project 674662b3-303d-428c-8532-9c8d97423172
2023-03-09 09:35:59,948 INFO [ProjectMetricsUpdateTask] Completed metrics update for project 36ffe57f-cc8d-489f-b64f-2f9a083a1096 in 00:03:596
2023-03-09 09:36:00,972 INFO [ProjectMetricsUpdateTask] Completed metrics update for project 674662b3-303d-428c-8532-9c8d97423172 in 00:04:620
2023-03-09 09:36:00,981 INFO [ProjectMetricsUpdateTask] Executing metrics update for project cb7d0960-8072-4e2b-af4e-d700d04301eb
2023-03-09 09:36:00,981 INFO [ProjectMetricsUpdateTask] Executing metrics update for project 883d614f-4628-48e9-a42a-7f2dfb9dbeed
2023-03-09 09:36:04,346 INFO [ProjectMetricsUpdateTask] Completed metrics update for project cb7d0960-8072-4e2b-af4e-d700d04301eb in 00:03:364
2023-03-09 09:36:04,788 INFO [ProjectMetricsUpdateTask] Completed metrics update for project 883d614f-4628-48e9-a42a-7f2dfb9dbeed in 00:03:806
2023-03-09 09:36:04,798 INFO [ProjectMetricsUpdateTask] Executing metrics update for project 89b0408c-4cb7-484d-bceb-097b8802f040
2023-03-09 09:36:08,162 INFO [ProjectMetricsUpdateTask] Completed metrics update for project 89b0408c-4cb7-484d-bceb-097b8802f040 in 00:03:363
2023-03-09 09:36:08,216 INFO [PortfolioMetricsUpdateTask] Completed portfolio metrics update in 00:16:754
```

### Proposed Behavior

It would be useful to be able to notify other systems whenever metric update happens on project, so we can know when was the last time update happened (and fetch any newer vulnerabilities).

### Checklist

- [X] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [X] I have checked the [existing issues](https://github.com/DependencyTrack/dependency-track/issues) for whether this enhancement was already requested