Plugin support for custom LLM providers now available

Hi all,

LangExtract now supports third-party model providers through a new plugin and registry infrastructure. You can integrate custom LLM backends (Azure OpenAI, AWS Bedrock, custom inference servers, etc.) without modifying core LangExtract code.

Please check out the example and documentation:
https://github.com/google/langextract/tree/main/examples/custom_provider_plugin

List of providers available: https://github.com/google/langextract/blob/main/COMMUNITY_PROVIDERS.md

Feel free to provide feedback or report any issues. Thank you!

@YunfanGoForIt @mariano @zc1175 @praneeth999 @JustStas @jkitaok