["Request"] Resilience Saga allow

What version are you currently using?

2.0.0-alpha.1

What would you like to see?

Similarly to nested transaction, I want to be able to automatically created nested Sagas. This can easily be done by injecting the `SagaContext` into the `CoroutineContext`, **if** nested sagas are configured, and any nested Saga could automatically be nested into the parent. This machinery should be internal to Resilience, so all new code to add this functionality should be private, and **not inlined**.

```kotlin
private class SagaContext(private val scope: SagaScope): CoroutineContext
private fun sagaContext(): SagaContext? = TODO()

fun sagaScope(
allowNested: Boolean = false,
block: SagaScope.() -> A
): Saga {
...
}
```

Start by locating the Resilience Saga implementation and the existing nested-transaction behavior, then inspect the SagaScope, sagaScope, and proposed SagaContext entry points. Done means nested Sagas are automatically attached to a parent only when nesting is enabled, with the added machinery private and not inlined.

Written by the indexing model from the issue text.