Add Service instance to `Invocation`

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Since Retrofit supports service interface hierarchies, the method() & arguments() of the Invocation are no more sufficient to distinguish the invocation. A method might be declared in a super class with Retrofit annotations but the subclass is the interface which gets created.

interface Super {
  @GET("/super") fun getSuper()
}

// This interface gets instantiated via Retrofit.create(Sub::class)
interface Sub: Super {
  fun getSuperFallback(error: Throwable) { ... }
}

The primary use case right now is to be able to support/implement Resilience4j’s CircuitBreaker fallback mechanism but there are other library use cases and it might open the door to many others.

[JakeWharton]

I think we’ll add both the class and the instance.

[efemoney]

soft bump ⬆️ @JakeWharton 😅