feat: add --custom-headers flag for custom endpoints

[kris3984]

Summary

Adds support for --custom-headers to pass custom HTTP headers to custom/BYOK OpenAI-compatible endpoints via LiteLLM.

Changes

• aider/args.py: Added --custom-headers CLI flag accepting repeated key-value pairs (KEY=VALUE, KEY:VALUE) or JSON strings.
• aider/models.py: Merged parsed headers into extra_params["extra_headers"] and forwarded them into model completion calls.
• tests/basic/test_models.py: Added unit tests covering header parsing, initialization, and completion forwarding.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Krishnakumar Chandran seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[kris3984]

Fixes #2841