Create a plan for CHLO

[burmako]

Open questions:

1. Are we going to have a spec for CHLO? (Almost certainly yes).
2. What kind of process / guarantees do we want to provide for CHLO? (Almost certainly the same as for StableHLO).
3. What ops go into CHLO?
4. With StableHLO, we're aiming to support all public features of HLO. What about CHLO - should we aim to support all public features of the Client HLO API?

[burmako]

When writing the previous comment, I was convinced that we should have a CHLO spec as well, similarly to the StableHLO spec. However, after thinking about this further, I'm realizing that this is not necessarily the case.

One seemingly viable option is treating CHLO similarly to how client HLO is treated - i.e. helper functionality that is useful for producers but is not reified in the operation set. This way, the StableHLO project will have less stuff to spec and less stuff to maintain, and StableHLO consumers will have less stuff to support.

This may run into logistical difficulties, because chlo-legalize-to-hlo currently lives in the MLIR-HLO repository, and it will likely be desirable to move it to this repository instead if we choose this course of action, but at the time of writing I think it's a viable alternative to consider.