Add stablehlo.async_start and stablehlo.async_done.

See openxla/stablehlo#2897 for context.

This CL introduces a `stablehlo.future` type and `stablehlo.async_start` and
`stablehlo.async_done`. It does not add any translation for them. That will
come in a later change. I also did not make `async_start` and `async_done`
variadic for now, even though some collectives are variadic. We can add support
for that later if we need it.

PiperOrigin-RevId: 890017379

Author: mwhittaker