Add first-class Xiaomi provider support

[Spherrrical]

Summary

• add Xiaomi as a native provider across config schema, CLI parsing, provider enums, and default Envoy upstream cluster
• extend fetch_models to retrieve Xiaomi models from https://api.xiaomimimo.com/v1/models using api-key auth
• add a Xiaomi MiMo integration demo under demos/integrations/xiaomi_mimo and link it from demos/README.md

[salmanap]

Couple of things

1/ I don't think we need a demo folder. We need to focus on just our docs as first-class
2/ We need to run our models_list script to pull all of their models so that developers can name them or run the wildcards

[adilhafeez]

Couple of things

1/ I don't think we need a demo folder. We need to focus on just our docs as first-class
2/ We need to run our models_list script to pull all of their models so that developers can name them or run the wildcards

@salmanap for demos folder I think we need a bigger change where we have separate repo hosting just our demos. That is something we have talked about in the past.