Add API_VERSION_TYPED_FFI to CustomCallOp

[ezhulenev]

Request description

XLA runtime supports typed custom calls with a new calling convention. This calling convention implemented in MHLO as mhlo::CustomCallApiVersion::API_VERSION_TYPED_FFI, and once it's proven to work for XLA and JAX it should be also added to StableHLO.

[GleasonK]

In the coming weeks we should have the compatibility infrastructure in a production ready state (v0.4.0), at which point we can start reviewing spec additions like this one.

I will be prioritizing this issue in early 2023.

Edit for clarity: I am not reviewing other spec additions currently, I am particularly interested in CustomCall with a dictionary because it overlaps with a feature we want to add in 2023. A dictionary allows us to do some interesting things with compatibility, namely if the attrs in it are StableHLO attributes, they can be upgraded/downgraded by our compatibility machinery, providing stability will less reliance on compiler back-ends than a string argument.

[ezhulenev]

I started working on re-writing XLA FFI (openxla/xla#6335), my current plan is to keep DictionaryAttr as a valid payload with some constraints on what can be a value. We should discuss what we want to standardize and what we can efficiently implement inside a runtime.