Allow @Source resolution for interface types

[reicheltp]

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• Consider creating a discussion on the discussion forum
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• Make sure the PR doesn't introduce backward compatibility issues
• Make sure to have sufficient test cases

Pull Request type

• Bugfix

Changes in this PR

Make the @Source attribute more robust by allowing to be bound to an interface / abstract class parameter.

• Updated SourceArgumentResolver to support assigning interface types by using isAssignableFrom.
• Added test cases for handling @Source with interface types.
• Extended GraphQL schema and test data to include Movie and Entertainment examples.

This PR solves the following problem:

Given a schema like this:

interface A {
   foo: String
}

type B extends A {
   foo: String
}

type C extends A {
   foo: String
}

Allows us to have a resolver using DgsDataFetchingEnvironment like this:

  @DgsData(parentType = "B", field = "foo")
  @DgsData(parentType = "C", field = "foo")
  public String getFoo(DgsDataFetchingEnvironment dfe) {
    A a = dfe.getSource();
    return service.resolve(a);
  }

But the implementation with @Source crashed with error: Invalid source type 'B'. Expected type 'A'. Regardless, that B is assignable to A.

  @DgsData(parentType = "B", field = "foo")
  @DgsData(parentType = "C", field = "foo")
  public String getFoo(@Source A a) {
    return service.resolve(a);
  }

Alternatives considered

• One resolver for each type -> bloating code.
• Sticking with dfe.getSource() -> boiler code.

[reicheltp]

@paulbakker Is there something missing, to get this reviewed?