How to use the engine as a general purpose notification service ?

Is there any example I can trigger the notification with a webhook or integrated into a message queue?
The idea is to use the engine as a general-purpose notification service.

Or is it achieved by adding custom Triggers? Like

```
apiVersion: v1
kind: ConfigMap
metadata:
name: argocd-notifications-cm
data:
trigger.on-api-call: |
- when: true
send: [api-call-recevied]
```
```
apiVersion: v1
kind: ConfigMap
metadata:
name: argocd-notifications-cm
data:
template.api-call-recevied: |
message: |
You have received an API call with message {{get-api-request-body_or_param}}.
```
So this engine can be used behind a rest microservice.

No repository files, tests, or entry points are named. Start by tracing how custom Triggers and the argocd-notifications-cm ConfigMap are processed, then inspect any existing REST, webhook, or message-queue integration. Done would require a decided, documented approach for invoking the engine as a general-purpose notification service.

Written by the indexing model from the issue text.