feat: multifile handling in `pin_upload`/`pin_download` #779

Job	Run time
Build Docs	1m 44s
Test a fork PR (no secrets)	0s
Test Posit Connect	5m 23s
Tests (3.8, ubuntu-latest, --workers 4 --tests-per-worker 1)	3m 0s
Tests (3.9, ubuntu-latest, --workers 4 --tests-per-worker 1)	2m 2s
Tests (3.10, ubuntu-latest, --workers 4 --tests-per-worker 1)	2m 46s
Tests (3.11, ubuntu-latest, --workers 4 --tests-per-worker 1)	2m 40s
Tests (3.12, ubuntu-latest, --workers 4 --tests-per-worker 1)	2m 0s
Tests (3.13, ubuntu-latest, --workers 4 --tests-per-worker 1)	1m 46s
Tests (ubuntu-latest, 3.8, requirements/minimum.txt)	15m 50s
Tests (macos-latest, 3.10, --workers 4 --tests-per-worker 1 -k pins/tests)	2m 16s
Tests (windows-latest, 3.10, -k pins/tests)	17m 7s
Preview Docs:	0s
Release to pypi	0s
Publish Docs	0s
	56m 34s

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