Feature: Entity type ID shorthand

[aaronsky]

This PR uses an experimental version of CreateAPI based on CreateAPI/CreateAPI#170 that enhances support around default values. Notably, the App Store Connect API schema doesn't use defaults at all, but there are several cases across the API where a required property is given a variant type with exactly one variant (such as type IDs). In this situation, there is an implied default value that can be provided, which subtly cleans up the call site in common situations.

For example, here is a code snippet before this PR:

_ = try await client.send(
    Resources.v1.betaTesterInvitations.post(
        .init(
            data: .init(
                type: .betaTesterInvitations,
                relationships: .init(
                    betaTester: .init(
                        data: .init(
                            type: .betaTesters,
                            id: tester.id
                        )
                    ),
                    app: .init(
                        data: .init(
                            type: .apps,
                            id: app.id
                        )
                    )
                )
            )
        )
    )
)

And here it is after the change:

_ = try await client.send(
    Resources.v1.betaTesterInvitations.post(
        .init(
            data: .init(
                relationships: .init(
                    betaTester: .init(data: .init(id: tester.id)),
                    app: .init(data: .init(id: app.id))
                )
            )
        )
    )
)