Decouple env var from apprunner.yaml

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**Tell us about your request**
Have an `apprunner.yaml` file with some of the core info such as the language runtime, etc and decoupling the env variable for convenience/secrecy which can be set independently via the console.

**Describe alternatives you've considered**
Discussion on https://github.com/aws-containers/hello-app-runner/issues/4

**Additional context**
NA

**Attachments**
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