Add oneTBB as a FetchContent dependency

[tom91136]

oneTBB works well when used as a CMake FetchContent dependency.
By doing this, TBB and the benchmark can be configured and compiled together which allows TBB to make better decisions based on the environment (e.g. TBB uses hwloc for NUMA awareness, if available during configuration).

[tom91136]

See https://github.com/UoB-HPC/cloverleaf_tbb/blob/23053cfc3d702bf289a07437577672f7f8fe7a9c/CMakeLists.txt#L9-L14 which integrates FetchContent TBB with CloverLeaf

[tomdeakin]

Done in a branch, will hit develop soon.