Split hash-based groupby into multiple smaller files to reduce build time

[PointKernel]

Description

This work is part of splitting the original bulk shared memory groupby PR #16619.

This PR splits the hash-based groupby file into multiple translation units and uses explicit template instantiations to help reduce build time. It also includes some minor cleanups without significant functional changes.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[PointKernel]

I'm open to further splitting this PR, but most of the changes are simply moving functions and classes into their own translation units and adding explicit instantiations.

[KyleFromNVIDIA]

Approved trivial CMake changes