global SSL configuration inheritance and SSL_CERT_FILE environment variable

[oliverkurth]

Summary

This PR introduces global SSL configuration inheritance and SSL_CERT_FILE environment variable support to tdnf, ensuring that SSL settings follow a canonical precedence hierarchy.

Previously, sslcacert had to be explicitly defined in every single .repo file, which was problematic for transparent MITM proxies. Additionally, tdnf did not respect the standard SSL_CERT_FILE environment variable.

Changes Made

• Global SSL Configuration: Added support for parsing sslcacert, sslclientcert, and sslclientkey in the [main] section of tdnf.conf. These settings are now inherited by all repositories unless explicitly overridden.
• Environment Variable Support: Added support for the SSL_CERT_FILE environment variable to override configuration files.
• Canonical Precedence Hierarchy: Enforced the standard configuration precedence (from highest to lowest priority):
  1. Command-line arguments (--setopt=sslcacert=... or --setopt=repoid.sslcacert=...)
  2. Environment variable (SSL_CERT_FILE)
  3. Per-repository configuration (.repo files)
  4. Global configuration (tdnf.conf)
• Memory Leak Fix: Fixed a pre-existing memory leak where pszSnapshotUrl and pszSnapshotFile were not being freed when destroying repository data structures. Consolidated TDNFFreeReposInternal and TDNFFreeRepos to prevent future drift.
• Test Suite Updates: Added a new pytest module (test_ssl_config.py) that spins up a local HTTPS server with self-signed certificates to verify the new inheritance and precedence logic. Updated TestUtils.run() in conftest.py to accept custom environment variables for testing.