CI: Trigger the cache_data workflow in PRs if cache files are added/deleted/updated

[seisman]

Description of proposed changes

Implement the idea proposed in #2937 (comment).

Changes in this PR:

• Move the download_test_data function from pygmt/helpers/testing.py into a separate file pygmt/helpers/caching.py and rename it to cache_data.py
• Update the cache_data workflow so that it's automatically triggered if pygmt/helpers/caching.py or .github/workflows/cache_data.yaml changes.

[codspeed-hq]

CodSpeed Performance Report

Merging #2939 will improve performances by 23.38%

_{Comparing cache-data (24bb4fe) with main (da36673)}

Summary

⚡ 1 improvements
✅ 63 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	cache-data	Change
⚡	test_sph2grd_no_outgrid	4.7 s	3.8 s	+23.38%