Implement two_hop_neighbors C API

[ChuckHastings]

Implements the two hop neighbors C API using the k_hop_nbrs algorithm.

This should unblock #2704

[seunghwak]

Looks good to me but should we better generalize this for k-hop neighbors (maybe not right now)?

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (branch-22.12@7387fbc). Click here to learn what that means.
Patch has no changes to coverable lines.

❗ Current head cfdabcb differs from pull request most recent head 6bceb70. Consider uploading reports for the commit 6bceb70 to get more accurate results

Additional details and impacted files

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[ChuckHastings]

Added #2918 to remind us to eventually expose the k-hop version of this.

[ChuckHastings]

@gpucibot merge