serverless: Inference service support customizing OpenAI API Base

[breezewish]

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• Inference service support customizing OpenAI API Base
• Fixed incorrect example of JINA

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[gemini-code-assist]

Summary of Changes

Hello @breezewish, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the flexibility of the inference service by allowing users to specify a custom API base for OpenAI embedding models, primarily to support Azure OpenAI deployments. It also refines existing documentation examples for both JINA AI and OpenAI embeddings, ensuring they reflect current best practices regarding vector dimensions and model choices. The changes aim to improve compatibility and provide clearer guidance for integrating various embedding services.

Highlights

• OpenAI API Base Customization: The inference service now supports customizing the OpenAI API Base URL, specifically enabling integration with Azure OpenAI services.
• JINA Embedding Example Correction: The example for JINA AI embeddings has been updated to correctly use a VECTOR(2048) dimension.
• OpenAI Embedding Model Update: The OpenAI embedding example has been upgraded to use the text-embedding-3-large model with a VECTOR(3072) dimension, replacing the previous text-embedding-3-small model and VECTOR(1536) dimension.
• Azure OpenAI Configuration Guide: New documentation has been added detailing how to configure and use Azure OpenAI embedding models by setting the TIDB_EXP_EMBED_OPENAI_API_BASE global variable.
• API Base Security Note: A security note has been included, clarifying that the TIDB_EXP_EMBED_OPENAI_API_BASE is restricted to Azure OpenAI or official OpenAI URLs, with arbitrary base URLs being forbidden.

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[gemini-code-assist]

Code Review

This pull request updates the documentation for Jina AI and OpenAI embeddings. It correctly updates the vector dimensions for a Jina AI example and introduces a new section for using Azure OpenAI, including how to customize the API base URL. The changes are valuable, but I've identified a few minor typos and areas for improved clarity in the new content. My review includes specific suggestions to address these points, ensuring the documentation is as clear and accurate as possible for users.