Add GPU-aware MPI halo exchange for MPMesh

[Shahrear2000]

Replaces #88, with the same changes opened from the polyMPO branch sj/gpu-aware-mpi-halo-exchange.

Main changes:

• Adds a GPU-aware MPI halo-exchange path in MPMesh, compiled when GPU_AWARE_MPI is defined.
• Keeps CPU-GPU staged (host copy) path as fallback.
• MPI buffers are allocated with plain cudaMalloc rather than Kokkos, because Kokkos' async memory pools are rejected by the CUDA IPC calls that Cray MPICH/GTL uses.

Build option:

• New CMake option polyMPO_ENABLE_GPU_AWARE_MPI. It is ON by default only when Kokkos has CUDA and Cray MPICH GTL is available; otherwise OFF.
• Override with -DpolyMPO_ENABLE_GPU_AWARE_MPI=ON/OFF.
• When OFF (for example CPU-only builds and GitHub CI), no CUDA or Cray code is compiled and the CPU-staged path is used.

Runtime notes:

• The GPU-aware path is used only when MPICH_GPU_SUPPORT_ENABLED=1; otherwise it falls back to the CPU-staged path.
• The selected path is printed once on rank 0 (polyMPO halo exchange: ...).

Testing:

• New testGPUAwareHaloExchange (4 ranks, owner → halo assignment, exact values checked).
• Passes on Perlmutter with 4 ranks in both modes: MPICH_GPU_SUPPORT_ENABLED=1 (GPU-aware) and =0 (CPU-staged).
• In GitHub CI the test is skipped: mpiexec -n 4 on the runner starts four independent single-rank processes, so the test exits with CTest's skip code (77).