Add support for UCX, CUDA and mpi_abi directly in MPICH easyblock

[Crivella]

• Add build option mpi_abi = False[/True] to turn the building of the MPI ABI on/off (with the related sanity check)
• Check if UCX is present at the EB level instead of in the easyconfigs
  • Could also have a build option to enable disable this
  • Could also automatically add the :ucx suffix to the device
• Added check for CUDA dependency to enable --with-cuda=... automatically

[Crivella]

Extensively tested through

• Modified MPICH Easyconfigs to work with EB from pr #3807 easybuild-easyconfigs#23239

[ocaisa]

LGTM