refactor: Add ProviderContext for a flexible storage directory

- Introduce ProviderContext class to decouple provider storage paths from absolute paths
- Add storage_dir attribute to StackRunConfig to accept CLI options
- Implement storage directory resolution with prioritized fallbacks:
  1. CLI option (--state-directory)
  2. Environment variable (LLAMA_STACK_STATE_DIR)
  3. Default distribution directory
- Standardize provider signatures to follow context, config, deps pattern
- Update provider implementations to use the new context-based approach
- Add comprehensive tests to verify state directory resolution

llama_stack/distribution/providers.py

@@ -23,7 +23,7 @@ class ProviderImplConfig(BaseModel):
     run_config: StackRunConfig
 
 
-async def get_provider_impl(config, deps):
+async def get_provider_impl(context, config, deps):
     impl = ProviderImpl(config, deps)
     await impl.initialize()
     return impl