[codegen models] add 'required' modifier in constructors when a field is non-nullable

### Description

The models generated by `amplify codegen models` won't compile because of the following error:

> The parameter 'id' can't have a value of 'null' because of its type, but the implicit default value is 'null'. [(Documentation)](https://dart.dev/tools/diagnostic-messages?utm_source=dartdev&utm_medium=redir&utm_id=diagcode&utm_content=missing_default_value_for_parameter#missing_default_value_for_parameter)
> Try adding either an explicit non-'null' default value or the 'required' modifier.

The generated field definition looks like this:
```dart
final String id;
```

And the generated constructor looks like this:
```
const Someclass._internal({this.id});
```

Since this field is non-nullable, the constructor should be generated like so:

```
const Someclass._internal({required this.id});
```

This causes us some overhead as we have to update the generated files manually each time.

We're on CLI version `12.0.3` and Flutter version `3.10.2`.

### Categories

- [ ] Analytics
- [ ] API (REST)
- [ ] API (GraphQL)
- [ ] Auth
- [ ] Authenticator
- [ ] DataStore
- [ ] Storage

### Steps to Reproduce

Have a model with a non-nullable field.

Run `amplify codegen models`.

Try compiling they project with the latest version of Flutter.

### Screenshots

_No response_

### Platforms

- [ ] iOS
- [ ] Android
- [ ] Web
- [ ] macOS
- [ ] Windows
- [ ] Linux

### Flutter Version

3.10.2

### Amplify Flutter Version

1.1.0

### Deployment Method

Amplify CLI

### Schema

_No response_