Adds litellm.proxy_auth to automatically obtain and refresh OAuth2/JWT
tokens when connecting to LiteLLM Proxy or any OAuth2-protected endpoint.
- Add ProxyAuthHandler for token lifecycle (obtain, cache, refresh)
- Add AzureADCredential wrapper for azure-identity credentials
- Add GenericOAuth2Credential for any OAuth2 provider (Okta, Auth0, etc)
- Auto-inject Authorization headers in completion() and embedding()
Closes#19834
b33e1e8019