feat: add Azure AD token-based authentication support

- Add azure_ad_token_provider parameter for token-based auth
- Add credential parameter for TokenCredential instances
- Create _TokenProviderCredential wrapper class for token providers
- Update authentication logic with priority: credential > token_provider > api_key
- Add support for base_url parameter as alternative to endpoint
- Update error message to reflect new authentication options
- Add comprehensive tests for all new authentication methods

Fixes #4018

Co-Authored-By: João <joao@crewai.com>