Support for multiple type in the same relationship

[daniel-m-dd]

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

When fetching a resource (say an article) I can get either

{
  "data": {
    "id": "1",
    "type": "articles",
    "attributes": { "title": "A" },
    "relationships": { "author": { "data": { "id": "1", "type": "human" } } }
  }
}

or

{
  "data": {
    "id": "1",
    "type": "articles",
    "attributes": { "title": "A" },
    "relationships": { "author": { "data": { "id": "xorg", "type": "bot" } } }
  }
}

Describe the solution you'd like

I would like to be able to Marshal/Unmarshal such a json api structure.

Describe alternatives you've considered

Tried to find a smart way to use StringIdentifier but not working.
I also tried to define twice the relationship field in my golang struct with the same json author key

type Article struct {
...
HumanAuthor        HumanAuthor `jsonapi:"relationship" json:"author"`
BotAuthor              BotAuthor `jsonapi:"relationship" json:"author"`
}

but the same json field cannot co-exist.

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