Create missing request-cache parent directories - #4047

Summary

• recursively create the configured request-cache directory before saving
• accept an already existing cache directory without a check-then-create race
• add regression coverage for saving and loading through a nested path with missing parents

Testing

PYTHONPATH=/tmp/lm-eval-test-deps:/tmp python3 -m pytest tests/test_cache.py -q
6 passed

PRE_COMMIT_HOME=/tmp/lm-eval-pre-commit pre-commit run --files lm_eval/caching/cache.py tests/test_cache.py
All hooks passed

Fixes #4046