Lower MPI to `stablehlo.custom_call` ops

[mofeing]

notes

• MPI types are not ABI stable, but fortunately there is MPItrampoline which provides some wrappers over them
  • MPIABI_Comm
  • MPIABI_Request
  • MPIABI_Op
• docs on how to register and use a custom_call:
  • https://jax.readthedocs.io/en/latest/ffi.html#c-interface
  • https://openxla.org/xla/custom_call

questions

• should we build libmpitrampoline into it or should we link on runtime?
• how do we register stablehlo.custom_calls? do we register them on some initialization phase in Reactant.jl or here?