Support SSM Secure references in AppRunner Service environment variables

### Name of the resource

AWS::AppRunner::Service

### Resource name

_No response_

### Description

Currently, referencing `SecureString` SSM Parameter Store values can only be done with a [subset of resources](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/dynamic-references.html#template-parameters-dynamic-patterns-resources). This is problematic when attempting to use these values to define runtime environment variables for an AppRunner Service. Without this support, sensitive values have to be either (A) stored in Parameter Store as plaintext Strings (suboptimal), or (B) those values must be resolved at synth-time, and the plaintext value is then exposed in the CloudFormation template (also not ideal).

While this issue specifically requests support for AppRunner Service environment variables, I'm sure this same issue plagues users looking to deploy sensitive values as environment variables to AWS Lambda et. al.

TIA!

### Other Details

Current result:
```
SSM Secure reference is not supported in: [AWS::AppRunner::Service/Properties/SourceConfiguration/ImageRepository/ImageConfiguration/RuntimeEnvironmentVariables]
```

Start with the reported AWS::AppRunner::Service path, especially SourceConfiguration/ImageRepository/ImageConfiguration/RuntimeEnvironmentVariables, and reproduce the current unsupported SSM Secure reference result. The issue names no repository files or tests; done would mean CloudFormation accepts the secure dynamic reference for these environment variables without exposing the plaintext value.

Written by the indexing model from the issue text.