feat: add passthrough_auth option for forwarding client Authorization header

[tqtensor]

Summary

This PR introduces the passthrough_auth option for model providers, allowing Plano to forward the client's Authorization header directly to upstream LLM services or proxies.

Changes

• Updated arch_config_schema.yaml to include passthrough_auth (boolean).
• Modified Python config generator to handle validation and warn if both passthrough_auth and access_key are provided.
• Updated Rust crates (common and llm_gateway) to implement the header forwarding logic.
• Added documentation in supported_providers.rst explaining the feature and use cases.
• Included a test configuration config/test_passthrough.yaml.

Verification Results

• Tested with a local LiteLLM instance where the virtual key was successfully forwarded.
• Verified that a warning is issued when both passthrough_auth and access_key are set.
• Ensured existing tests pass.

Closes #686

[salmanap]

Thank you @tqtensor - this is a well thought out feature. Thanks for your contribution. We will take this change in. @adilhafeez will run the workflows for testing and perhaps offer a couple of small code review comments

[adilhafeez]

Only one minor comment - looks good overall. Thanks for your contribution @tqtensor

[adilhafeez]

Created another PR to force run gh actions with api keys - #689

[adilhafeez]

hey @tqtensor I just pushed a fix to your PR

[tqtensor]

Thanks @salmanap and @adilhafeez for the quick review and help! I'm back online now and will check the updates.

[salmanap]

Btw @tqtensor would love for you to join our discord. Love contributions like these and hope to keep on building together