Allow N:1 mapping of union keys to union constructor

Is your feature request related to a problem? Please describe.

From what I have seen, union constructors map 1:1 with a union key. There are cases where I'd like to be able to have multiple union keys map to the same union constructor.

To support this use-case in the past I have duplicated the union class constructor with the alternate union key. This works, but it becomes extremely verbose and repetitive and creates hard-to-maintain freezed classes. For example, if you want to change one property you have to make sure all N duplicated constructors get updated in the same way

It would be ideal if union keys could map N:1 to union constructors such that this duplication were not necessary.

Bonus: Would be great if you could 'extend' a particular union constructor whereby it would automatically inherit the properties of a particular constructor and allow you to add additional properties to it. In the past I've had to similarly copy/paste the entire constructor just to add 1-2 additional properties. I know this could be solved via data modelling but that's not always possible when you're consuming a 3rd party api

Describe the solution you'd like Perhaps an approach like this may be ideal for maintaining backwards compatibility

  @JsonSerializable()
  @FreezedUnionValue('UNION_KEY_1')
  @FreezedUnionValue('UNION_KEY_2')
  @FreezedUnionValue('UNION_KEY_3')
  const factory MyObject.someUnionCase({
    required String id,
    required String name,
  }) = _MyObject;

Alternatively adding a new decorator could also work

  @JsonSerializable()
  @FreezedUnionValues([
     'UNION_KEY_1', 
     'UNION_KEY_2', 
     'UNION_KEY_2'
  ])
  const factory MyObject.someUnionCase({
    required String id,
    required String name,
  }) = _MyObject;

Describe alternatives you've considered As described above I've previously duplicated the union cases which is not ideal as it creates a lot of boilerplate and makes map(...) functions duplicative unnecessarily. It also leads to a lot more generated code which could be avoided entirely if multiple keys could just map to a single union case

Additional context Add any other context or screenshots about the feature request here.