feat: Add LLM Provider support in plugin runtime

- Introduced LLM Provider declarations in plugin manifests, allowing plugins to specify their LLM capabilities.
- Implemented validation for LLM Provider declarations to prevent duplicates and conflicts.
- Enhanced the PluginRunner to handle LLM Provider invocation requests, enabling plugins to interact with LLM Providers seamlessly.
- Added a ClientRegistry to manage LLM Provider registrations and ensure no conflicts arise between different plugins.
- Created a PluginLLMClient to facilitate communication with LLM Providers through the plugin runtime.
- Developed tests to ensure proper registration and conflict handling of LLM Providers.

src/plugin_runtime/runner/plugin_loader.py

@@ -54,6 +54,7 @@ class PluginMeta:
         self.capabilities_required = list(manifest.capabilities)
         self.dependencies: List[str] = list(manifest.plugin_dependency_ids)
         self.component_handlers: Dict[str, str] = {}
+        self.llm_provider_handlers: Dict[str, str] = {}
 
 
 class PluginLoader: