Add Open MPI to TheRock

[marbre]

Some RCCL-tests require a Message Passing Interface (MPI) implementation to run in a multi-node set up. These tests get enabled through USE_MPI , which is set via THEROCK_ENABLE_MPI as soon as #1283 has landed.

We eventually want to vendor Open MPI as an MPI implementation with TheRock.

Open MPI can be build with ROCm-aware support (https://docs.open-mpi.org/en/main/tuning-apps/networking/rocm.html) if UCX gets included in TheRock as well and if the latter is built with ROCm support.