gc: add dry option and log num removed objs

[dberenbaum]

Requires treeverse/dvc-data#441 (and need to add a version bump here).

Closes #9330

Adds gc --dry option and reports the number of files removed for each cache.

Before this PR (a file is removed here but it look like nothing happened):

$ dvc gc -w
WARNING: This will remove all cache except items used in the workspace of the current repo.
Are you sure you want to proceed? [y/n]: y
No unused 'local' cache to remove.
No unused 'legacy' cache to remove.

After this PR:

$ dvc gc -w
WARNING: This will remove all cache except items used in the workspace of the current repo.
Are you sure you want to proceed? [y/n]: y
Removed 1 objects from repo cache.
No unused 'local' cache to remove.
No unused 'legacy' cache to remove.

[dberenbaum]

This assertion was passing even before running gc. It wasn't actually retrieving oids from bad_cache because it was checking if they exist in the fs, and here they only exist in the odb.

[codecov]

Codecov Report

All modified lines are covered by tests ✅

Files	Coverage Δ
dvc/commands/gc.py	83.87% <100.00%> (+0.26%)	⬆️
dvc/repo/gc.py	92.10% <100.00%> (+0.21%)	⬆️
tests/func/test_gc.py	100.00% <100.00%> (ø)
tests/unit/command/test_gc.py	100.00% <ø> (ø)

📢 Thoughts on this report? Let us know!.

[sjawhar]

Oh my goodness, thank you for this! 🙏