r.resamp.filter: implement parallelization with OpenMP

[aaronsms]

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Checklists before merging:

• [x] code review
• [x] CI passes
• [ ] performance section in documentation
• [ ] confirm values in test are from the old version (run new tests with old code)
• [ ] run tests without OpenMP
• [ ] visual check of results with custom data ("looks good" with non NC SPM dataset)
• [ ] check that it works with really large data (16B cells)
• [ ] run multi-core benchmark (no degraded performance with many threads)
• [ ] run one core benchmark on many resolutions or many cell
• [x] rebase to main