[Feature Request] SNS trigger for "pcluster build-image"

**_Background:_**

AWS blog article [1] details on how users can build their own ParallelCluster AMI using the **pcluster** commands:

```
pcluster build-image --image-id myFirstCustomImage --image-configuration my-build-config.yaml
```

And then check the status of the image build using:

```
pcluster describe-image --image-id myFirstCustomImage --query 'ec2AmiInfo.amiId'
```

The above commands does introduce new possibility to build a custom ParallelCluster AMI, however the steps are very manual and doesn't aid any automation requirements to use the ImageId elsewhere, for example (maybe storing the ImageId in SSM Parameter Store).

**_Ask:_**

Can ParallelCluster introduce something like the below to aid automation?
1. "triggers" that can invoke SNS or Lambda function?
2. OR "sns topics" to the image build step

References:
[1] https://aws.amazon.com/blogs/hpc/custom-amis-with-parallelcluster-3/