Add HPX fork_join_executor implementation for intra-node parallelism

[guptapratykshh]

This PR adds a new HPX implementation (hpx_task_bench.cpp) that uses hpx::execution::experimental::fork_join_executor for efficient intra-node task graph execution, complementing the existing distributed implementation.

[elliottslaughter]

Thanks for submitting this PR. In generally I'm happy to take new implementations, but I'd like to make sure that the build gets integrated and that CI passes with this implementation.

I may need to put in some work from my end to get the rest of the CI to work again, but you don't need to worry about that. If you can just get the various HPX implementations integrated that'll be great.

[guptapratykshh]

Thanks, @elliottslaughter I have just pushed an update that integrates the new hpx_task_bench implementation into the test_all.sh suite so it can be verified alongside the other backends. I also updated the CMake configuration to handle both the distributed and standalone versions (making MPI optional)