Refactor prepack buffer code (#16280)
### Description
1. Use IAllocatorUniquePtr to replace BufferUniquePtr. It will ensure
the deleter is always right.
2. Change some std::unique_ptr to std::optional
3. Bypass Arena allocator when allocating the prepack buffers for mlas.
In this special case, Arena doesn't help any. And this change is just an
internal implementation change, it doesn't affect our public interface.
