serverless-export-env functionality

### Is this feature request related to a new or existing Amplify category?

auth, storage, function, api, predictions, xr

### Is this related to another service?

_No response_

### Describe the feature you'd like to request

I was hoping to be able to export environment variables in a similar fashion as to how [serverless-export-env](https://www.npmjs.com/package/serverless-export-env) works

### Describe the solution you'd like

I guess it would be a command like `amplify export env` that would export the current environment outputs to a .env file. For example, if I have a custom cdk resource with the following output it woudl create a .env file with `snsTopicArn=someValue`

```
new cdk.CfnOutput(this, 'snsTopicArn', {
value: topic.topicArn,
description: 'The arn of the SNS topic',
});
```

### Describe alternatives you've considered

Not sure how this might tie into the existing aws-exports.js

### Additional context

_No response_

### Is this something that you'd be interested in working on?

- [ ] 👋 I may be able to implement this feature request
- [ ] ⚠️ This feature might incur a breaking change

Start at the CLI entry point for the proposed `amplify export env` command and compare its output model with `aws-exports.js`. Define how current environment outputs, including the custom CDK `CfnOutput` named `snsTopicArn`, should map to a `.env` file; done means the command exports those values consistently without leaving the existing configuration unclear.

Written by the indexing model from the issue text.