(gen2-migration) `generate` command should handle secrets in function definitions

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

function

### Is this related to another service?

_No response_

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

When a function is configured with a secret in Gen1, the `generate` command doesn't currently convert it to the appropriate Gen2 APIs. For example, if a function is configured with a secret called `MY_SECRET`, we will generate:

```ts
import { defineFunction } from '@aws-amplify/backend';

const function = defineFunction({
environment: {
MY_SECRET: "/amplify//main/AMPLIFY__MY_SECRET"
}
})
```

This happens because in Gen1, the CLI will store the secret value as a `SecureString` SSM parameter and configure the path to this parameter as the value of the environment variable. The function itself is then responsible for fetching the secret value during runtime using the `@aws-sdk/client-ssm` package.

While this will continue to work in Gen2, it is not the native way Gen2 functions should handle secrets.

### Describe the solution you'd like

In Gen2, the secret values are automatically fetched when the function is loaded. It is then available to the function code by accessing the corresponding env variable (i.e `process.env.MY_SECRET`). So, what we should generate is:

```ts
import { defineFunction, secret } from '@aws-amplify/backend';

const function = defineFunction({
environment: {
MY_SECRET: secret("MY_SECRET")
}
})
```

### Notes

- For this to work, the `MY_SECRET` secret needs to be defined in the amplify console for the appropriate app. The customer is still expected to define those secrets manually prior to deployment.
- Customers are also expected to manually change their code to start using `process.env.MY_SECRET` instead of fetching it explicitly from SSM.

### Describe alternatives you've considered

None

### Additional context

https://github.com/aws-amplify/amplify-cli/blob/gen2-migration/GEN2_MIGRATION_GUIDE.md#post-generate--function-secrets

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

- [ ] 👋 I may be able to implement this feature request

### Would this feature include a breaking change?

- [ ] ⚠️ This feature might incur a breaking change