feat: add AIML API provider

[D1m7asis]

What does this PR do?

This PR adds support for the AI/ML API provider — a flexible, OpenAI-compatible platform that offers access to 300+ models (e.g., GPT-4o, Claude, Gemini, DeepSeek). It enables users to configure and use aimlapi/<model> as a valid chat_model within Langroid.

✅ Changes included:

• AIML_API_KEY added to .env-template
• Docs updated (llm-usage-options.md, non-openai-llms.md, local-llm-setup.md, supported-models.md)
• aimlapi/ support added in openai_gpt.py for config parsing and base URL override
• Custom headers X-Title and Referer added for tracking
• Sync and async test cases for several aimlapi/* models
• Provider key logic extended in test_keys()

📘 Documentation

Documentation added in the following files:

• llm-usage-options.md
• non-openai-llms.md
• local-llm-setup.md
• supported-models.md

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🧪 Verification

All added tests (test_llm_aimlapi, test_llm_aimlapi_async) pass and return expected values for AIMLAPI models.

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Let me know if anything should be adjusted or restructured!

[D1m7asis]

From https://github.com/langroid/langroid/issues/860

[pchalasani]

From #860

Thank you. I will try to review it in the next week or so.

[D1m7asis]

Hi! @pchalasani Can you pls look at the pr? Thanks!

[pchalasani]

Hi! @pchalasani Can you pls look at the pr? Thanks!

Thanks, will review in a few days.

[OctavianTheI]

Hi @pchalasani! Let me know if any changes need to be implemented after your review, we'll do those within 1-2 days