Suspend support in interceptors

[yschimke]

A quick riff on #7904 to provoke discussion

Detection is by two mechanisms

1. Use a thread local set in AsyncCall.executeOn, if the interceptor strictly on the same thread, this works. But not swapping dispatchers.
2. Use chain.callFactory which never actually enqueues.

We should look at what is possible on SuspendingInterceptor, if we say that this support relies on using it, and all frameworks (wire, retrofit) are suspending, then maybe we can carry some information in the CoroutineContext to detect this, and pick it back up again in executeAsync?