fix: expose s3-specific endpoint config

[Guflly]

Fixes #753.

This adds an s3_endpoint option so callers can explicitly override AWS_ENDPOINT_URL_S3. The alias round-trips through config and pickling, and the regression covers the environment precedence case.

Draft #755 touches the same config-key conversion code, so this may need a small rebase if it lands first.

Tests:

• cargo test --all --all-features
• cargo clippy on pyo3-object_store
• Ruff and Pyright
• endpoint regression against a local HTTP server

[kylebarron]

Thanks