[VitisAI] Add External EP Loader

[BoarQing]

[VitisAI] Add External EP Loader

Description

This PR introduces a dynamic external execution provider loading mechanism for the VitisAI execution provider, enabling runtime loading of alternative execution providers through a plugin-style architecture.

Key Changes

1. New External EP Library Infrastructure (global_api.cc)

• Added ExternalEpLibaray class to dynamically load external execution provider libraries at runtime
• Implemented complete library lifecycle management (loading, unloading, symbol resolution)
• Added global registry (g_external_ep_libaries) with caching to avoid redundant library loading
• Created CreateExecutionProviderFromAnotherEp() function to instantiate execution providers from external libraries

Implementation Details:

• Simplified symbol resolution: Only resolves the essential GetProvider symbol (required)
• Removed optional symbols: No longer attempts to resolve CreateEpFactories or RyzenAI_SetSessionOptions
• Lazy initialization pattern with Ensure() method
• Safe cleanup with Clear() method and proper error handling
• Platform-agnostic library loading using LIBRARY_PREFIX and LIBRARY_EXTENSION macros

2. API Extension (global_api.h)

• Declared new public function: CreateExecutionProviderFromAnotherEp()
• Added required includes:
  • core/framework/execution_provider.h for IExecutionProvider interface
  • <memory> for smart pointer support

3. Factory Integration (vitisai_provider_factory.cc)

• Integrated external EP loading into the VitisAI provider factory workflow
• Added provider option check for external_ep_libray key
• Logic Flow:
  1. Check if external_ep_libray option is specified
  2. If yes, load and return the external execution provider
  3. If no, create and return standard VitisAI execution provider