feat(RHI): 实现 RHIFence 抽象基类

- 新增 RHIFence 抽象基类
- D3D12Fence 继承 RHIFence
- OpenGLFence 继承 RHIFence
- 文档更新 RHIFence 差异处理策略

engine/src/RHI/D3D12/D3D12Fence.cpp

@@ -44,7 +44,7 @@ void D3D12Fence::Wait(uint64_t value) {
     }
 }
 
-uint64_t D3D12Fence::GetCompletedValue() {
+uint64_t D3D12Fence::GetCompletedValue() const {
     return m_fence->GetCompletedValue();
 }