aws-apprunner-alpha: missing option for referencing environment variables via SM parameters

### Describe the bug

There's no option in the current `Service` L2 construct for [referencing environment variables](https://docs.aws.amazon.com/apprunner/latest/dg/env-variable.html) using `SecureString` parameters in Systems Manager.

### Regression Issue

- [ ] Select this option if this issue appears to be a regression.

### Last Known Working CDK Version

_No response_

### Expected Behavior

I expected to be able to reference environment variables from SM Parameter Store like from AWS Console.


### Current Behavior


You only have the following two options available within the `Service` construct props:

- `environmentVariables` ➜ equivalent to `Plain text` in AWS Console
- `environmentSecrets` ➜ equivalent to `Secrets Manager` in AWS Console

### Reproduction Steps

See above.

### Possible Solution

Introduce a new `environmentParameters` option following the same principle as the other two props.

### Additional Information/Context

_No response_

### CDK CLI Version

2.174.0

### Framework Version

_No response_

### Node.js Version

22.12.0

### OS

Ubuntu 24.04.1

### Language

TypeScript

### Language Version

_No response_

### Other information

_No response_

Start at the App Runner Service L2 construct and compare the existing environmentVariables and environmentSecrets props with the linked AWS environment-variable documentation. Add the proposed environmentParameters capability for SecureString Systems Manager Parameter Store references, then verify the synthesized configuration alongside the existing environment options.

Written by the indexing model from the issue text.