[Feature Request] Case insensitive parsing of enums

Due to inconsistencies in the data sources my app needs to handle various variants of enum representations e.g.:

```json
{
"myEnum": "VALUE"
}
```

```
{
"myEnum": "value"
}
```

I wish there was a way to tell json_serializable to ignore the case differences and parse these values to `MyEnum.value`.

The API I would love to see could be:

In Dart:

```dart
JsonKey(ignoreCase: true)
final MyEnum myEnum;
```

and the generated code would then call `toString().toLowerCase()` on the value being passed to `enumDecodeNullable`:

```dart
$checkedConvert(
json,
'myEnum',
(v) => _$enumDecodeNullable(_$MyEnumEnumMap, v?.toString().toLowerCase(),
unknownValue: MyEnum.unknown)) ??
MyEnum.unknown,
```