Futures: Add extensions to get OS kernel event handles from Futures

[kainino0x]

This is the theoretically-optimal way to integrate into existing event loops in applications and runtimes.

This was described in the original design doc here:
https://docs.google.com/document/d/1qJRTJRY318ZEqhK6ryw4P-91FumYQfOnNP6LpANYy6A/edit?tab=t.0#heading=h.o77zi44jdz5a