Generated JsonMap object by discovery_apis_generator does not store the value

**Reposteps**

1. Have an discovery document with an object that contains JsonMap.
2. Use the discovery_apis generator
3. Notice the JsonMap object is created:

```dart
class Foo{
core.String? bar;
JsonMap? baz;
```

```dart
class JsonMap {
JsonMap();

JsonMap.fromJson(
// ignore: avoid_unused_constructor_parameters
core.Map _json);

core.Map toJson() => {};
}
```

There is no way to retrieve the actual value of the JsonMap and casting throws an cast error.

**Desired solution**
A way to retrieve the values of the JsonMap. For example the object can look like:

```dart
class JsonMap {
JsonMap();

JsonMap.fromJson(
// ignore: avoid_unused_constructor_parameters
core.Map _json) {
_map = _json as core.Map?;
}

core.Map? get raw => _map;

core.Map? _map;
core.Map toJson() => {};
}
```

Btw if you agree with my proposal I can create a pr.