custom_resources: onEvent response disagreement on if PhysicalResourceId is required

### Describe the issue

When implementing a CustomResource, the onEvent handler must return some json.
There is disagreement about if PhysicalResourceId is required.

This repo [says](https://github.com/aws/aws-cdk/blob/a7de7feb6a14658ec25f4cfda434d5e1d69157d2/packages/aws-cdk-lib/custom-resources/README.md?plain=1#L157) it's **not** required.

The [CloudFormation guide](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/crpg-ref-responses.html#crpg-ref-responses-physicalresourceid.title) says that it **is** required.

The [typescript definitions](https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/aws-lambda/trigger/cloudformation-custom-resource.d.ts#L50) say it **is** required, but looks like it was *trying* to make it only required if it's Update or Delete and then they did all event types.

I can make PRs once I know what it should be.
Though I don't know where the CloudFormation guide source is.

### Links

* https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.custom_resources-readme.html#handling-lifecycle-events-onevent
* https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/crpg-ref-responses.html#crpg-ref-responses-physicalresourceid.title

Compare the custom-resources README section linked in the issue with the CloudFormation response guide and the DefinitelyTyped cloudformation-custom-resource.d.ts definition. Determine the authoritative PhysicalResourceId requirement for each lifecycle event, then align the documented guidance and relevant type definition or record the agreed clarification.

Written by the indexing model from the issue text.