Implement function calling and plugin API with OAuth support

[bgervin]

PRD: Function Calling and Plugins

Objective

Implement function calling for registered API-based plugins, including OAuth authentication.

Tasks

• Support plugin registration and discovery.
• Implement function calling for plugins.
• Add OAuth support (interactive and key/config file options).
• Document OAuth configuration approaches (key file, config file, etc.).

Acceptance Criteria

• Plugins can be called via registered APIs.
• OAuth can be configured and used interactively or via files.