[client] Add proxy certificate support for HTTPS connections (#12177)

[maelv-filigran]

Proposed changes

• Added automatic HTTPS proxy certificate handling in OpenCTI client-python
• Implemented _setup_proxy_certificates() method that detects HTTPS_CA_CERTIFICATES environment variable and combines proxy certificates with system certificates
• Added support for enterprise proxy environments by creating secure temporary certificate bundles
• Configured SSL verification for both requests library and urllib through environment variables

Related issues

• Resolves proxy connectivity issues for Python connectors in enterprise environments
• Generalizes the connector proxy certificate fix to benefit all Python-based connectors
• OCTI #12177

Checklist

• I consider the submitted work as finished
• I tested the code for its functionality
• I wrote test cases for the relevant uses case
• I added/update the relevant documentation (either on github or on notion)
• Where necessary I refactored code to improve the overall quality

Further comments

This implementation provides a centralized solution for HTTPS proxy certificate handling in enterprise environments. The solution:

• Zero Configuration: Automatically activates when HTTPS_CA_CERTIFICATES environment variable is provided
• Enterprise Ready: Combines proxy certificates with system certificates from multiple Linux distributions (Debian/Ubuntu, RHEL/CentOS, Alpine/BSD)
• Backwards Compatible: No impact on existing configurations
• Secure: Uses temporary directories with proper permissions and graceful error handling
• Universal: Benefits all Python connectors automatically without individual modifications