[FEA] Support <4 byte types in GroupBy.apply with engine='jit'

[brandon-b-miller]

Is your feature request related to a problem? Please describe.
During #13729 we were able to round out the 4 and 8 byte dtypes supported by the groupby JIT engine, however shorter types such as int8 and int16 couldn't be added due to the missing support for those types from certain libcu++ features we rely on in the shim library.

Describe the solution you'd like
We should provide block level implementations for these types in c++ that don't rely on the particular libcu++ features.

Describe alternatives you've considered
We could also wait for libcu++ to provide things like atomic_ref for shorter types.

cc @bwyogatama @PointKernel

[PointKernel]

Related to NVIDIA/cccl#1024