Allow annotated field with JsonKey.unknownEnumValue to be of type Map

At present, the #585 and PR #691 allows us to annotate `unknownEnumValue` fields of type enum or on Iterable, List, or Set instances of an enum type. Is is possible to support Map ?

Example:

```
@JsonKey(unknownEnumValue: YourEnum.yourUnkownValue)
final Map minWithdrawalAmount;
```

This is currently not possible. Enclosed error shown in terminal when generate the code

```
Error with `@JsonKey` on the `minWithdrawalAmount` field. `unknownEnumValue` can only be set on fields of type enum or on Iterable, List, or Set instances of an enum type.
package:yourfile.dart:19:22
╷
19 │ Map minWithdrawalAmount;
│ ^^^^^^^^^^^^^^^^^^^
╵
```