`ArrayPromise` could be replaced by an ALTREP

[sebffischer]

I think it's nice that the PJRTBuffer represents both the promise and the value and it supports the await() interface.
For PJRTArrayPromise this is not possible because we don't control the R array object. However, we could make it an ALTREP so the interface would be standardized (we could also await() the result of as_array_async()). What do you think @dfalbel ? Maybe it's not worth it ...

[dfalbel]

That would be nice indeed. as_array could always return an ALTREP array asynchronously.
Maybe not that hard to implement with an agent.