Allow environment variables from Cfn template and apprunner.yaml

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**Tell us about your request**
Setting up AppRunner Service via IasC provides well known benefits. Right now, it is possible to set environment variables like table names or other dynamic parameters from a Cfn template.
This is only possible, if the template does not use the `apprunner.yaml`, otherwise configuration within `CodeConfigurationValues` are ignored.

It would be nice to be able to overwrite environment variables of the `apprunner.yaml` with values from CloudFormation.

**Describe alternatives you've considered**
Right now we provide `CodeConfigurationValues` and skip the benefits of a pre or post build command.