Add custom ops library_path to EP metadata (#25830)
## Summary
Adds EP metadata library path support to enable custom ops DLL
registration with proper path resolution.
## Changes
- Added `library_path` metadata key to EP metadata infrastructure
- Pass resolved library path directly to `EpLibraryProviderBridge`
constructor
- Simplified implementation per reviewer feedback (removed virtual
method complexity)
- Added `#include <utility>` for std::move compliance
## Purpose
Enables downstream applications (like onnxruntime-genai) to resolve
relative custom ops library paths using EP metadata, improving DLL
registration reliability.
## Files Modified
- `plugin_ep/ep_factory_provider_bridge.h`
- `plugin_ep/ep_library.h`
- `plugin_ep/ep_library_plugin.h`
- `plugin_ep/ep_library_provider_bridge.cc`
- `plugin_ep/ep_library_provider_bridge.h`
- `utils.cc`
