feat(api): Add support for custom HTTP headers via config (which can point to environment variables)

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Description

• Add custom_headers section to host config with support for direct header values and values loaded from environment variables
• Apply headers to both GitLab client operations and direct API calls
• Add debug redaction for sensitive headers (Proxy-Authorization, Cf-Access-Client-Secret)
• Allows support for authentication proxies like Google IAP and Cloudflare Access
• Add test coverage for all header scenarios
• Update README with usage examples and authentication proxy patterns

Related Issues

Fixes #1037 (closed)

How has this been tested?

Unit tests, and manually by using it to perform operations on my IAP-protected self hosted Gitlab instance.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap