Add a way to get ProtoEnum values

Enums should support a `values` field that acts like the proto generator and provides a list of possible values e.g.

```proto
enum Color {
COLOR_UNSPECIFIED = 0;
COLOR_RED = 1;
COLOR_GREEN = 2;
COLOR_BLUE = 3;
}
```

to

```dart
static const List values = [
COLOR_UNSPECIFIED,
COLOR_RED,
COLOR_GREEN,
COLOR_BLUE,
];
```