Support adding marker interface to oneOfs.

[cjbooms]

While the discriminator property is needed to enable full polymorphism with Jackson, it should not be a blocker to applying the sealed interface to classes that deserve it.

This PR allows the interface to be applied, without the Jackson polymorphism annotations:

public sealed interface State

public data class StateA(
    @get:JsonProperty("status")
    @get:NotNull
    public val status: String,
) : State

public data class StateB(
    @get:JsonProperty("mode")
    @get:NotNull
    public val mode: String,
) : State

Jackson won't be able to figure out which object to deserialise to, but it is beneficial to have the marker interface in code

[cjbooms]

@pschichtel I have started using the SEALED_INTERFACES_FOR_ONE_OF feature you added in some work projects recently, and I have to say I am a big fan. 💯

Possibly I should make it the default behaviour in a version soon.

This PR extends support even more for a different use case I have.

[cjbooms]

Here I am not sure whether to reference State when the Polymorphic annotations are missing, or whether I should have it set to Any to ensure deserialisation is not broken

[averabaq]

Nor I. Well, the model generated is correct but having a model that fails at deserialization defeats the purpose 🤔 I'd slightly lean for Any, but I'm on the fence.

[cjbooms]

OK, changed to Any. That at least preserves the existing behaviour better. If anyone comes looking for it, we can add a new configuration option to allow them to opt-in

[averabaq]

Gotcha! 👍

[averabaq]

Nor I. Well, the model generated is correct but having a model that fails at deserialization defeats the purpose 🤔 I'd slightly lean for Any, but I'm on the fence.