Enable RCSB feature for Linux Python wheels

[HFooladi]

Summary

Currently, Linux Python wheels are built without the RCSB feature due to OpenSSL cross-compilation issues in manylinux containers. This means Linux users cannot use download_structure() and rcsb_search() functions.

Current Behavior

Platform	RCSB Feature
macOS	✅ Available
Windows	✅ Available
Linux	❌ Not available

Root Cause

The reqwest crate uses native-tls by default, which requires OpenSSL. Cross-compiling OpenSSL for Linux aarch64 in manylinux containers fails because:

1. before-script-linux in maturin-action runs on the host, not inside the container for cross-compilation
2. native-tls-vendored requires Perl modules (IPC::Cmd, FindBin) not available in manylinux containers
3. rustls-tls alternative fails because ring crate doesn't support aarch64 cross-compilation

Potential Solutions to Investigate

1. Use rustls-tls with aws-lc-rs backend

• ring is being replaced by aws-lc-rs in many projects
• May have better cross-compilation support
• Requires reqwest feature configuration change

2. Custom manylinux Docker image

• Pre-install OpenSSL development headers
• Pre-install required Perl modules
• Use as custom container in CI

3. Use zig for cross-compilation

• cargo-zigbuild handles cross-compilation better
• May resolve OpenSSL linking issues

4. Separate RCSB into optional runtime dependency

• Ship Linux wheels without RCSB compiled in
• Provide separate pdbrust-rcsb package that users can install if needed
• Would require Python-side HTTP implementation

5. Wait for maturin/manylinux improvements

• Track upstream issues for better OpenSSL support
• manylinux_2_28 might have better tooling

References

• maturin-action: https://github.com/PyO3/maturin-action
• reqwest TLS options: https://docs.rs/reqwest/latest/reqwest/#optional-features
• ring cross-compilation: https://github.com/briansmith/ring/issues/1]+

Acceptance Criteria

• Linux x86_64 wheels include RCSB feature
• Linux aarch64 wheels include RCSB feature
• All existing tests pass
• CI builds remain stable