Support MongoDB cache credentials outside URI

[mindbomber]

Fixes #2522.

Summary

• Resolve MongoDB cache settings from --mongo-uri, HELM_MONGODB_URI, or mongoUri in credentials.conf.
• Allow MongoDB username/password to be supplied via HELM_MONGODB_USERNAME / HELM_MONGODB_PASSWORD or mongoUsername / mongoPassword in credentials.conf.
• Pass MongoDB credentials to MongoClient separately and redact credentials from cache config representations and cache stats keys.
• Document the new credentials and environment variable options.

Verification

• python -m compileall src\helm\common\cache_backend_config.py src\helm\common\cache.py src\helm\common\mongo_key_value_store.py src\helm\benchmark\run.py src\helm\proxy\server.py
• $env:PYTHONPATH='src'; $env:PYTHONUTF8='1'; python -m pytest -o addopts='' src\helm\common\test_cache_backend_config.py -q
• python -m ruff check src/helm/common/cache_backend_config.py src/helm/common/cache.py src/helm/common/mongo_key_value_store.py src/helm/benchmark/run.py src/helm/proxy/server.py src/helm/common/test_cache_backend_config.py
• python -m black --check src/helm/common/cache_backend_config.py src/helm/common/cache.py src/helm/common/mongo_key_value_store.py src/helm/benchmark/run.py src/helm/proxy/server.py src/helm/common/test_cache_backend_config.py
• git diff --check