Add support for PhysicalResourceId reference

[pflorek]

When I provide a custom PhysicalResourceId in the onCreate GithubApiCall, then maybe I want to reference it in the onUpdate GithubApiCall.

See https://github.com/pepperize/cdk-github/blob/main/API.md#physicalresourceidoptional-

This mimics the very same behavior of AWS CDK - custom_resources -PhysicalResourceIdReference