feat(bindings/cpp): eliminate indirect pointers by manually managing memory

[deadlinefen]

Eliminate indirect pointers by manually managing the memory of returned classes in lib.rs

Which issue does this PR close?

nop.

Rationale for this change

The previous type erasure implementation using unique_ptr introduced an indirection pointer, resulting in additional overhead.
This was previously mentioned in the earlier PR #6041.

What changes are included in this PR?

1. Modifying lib.rs to return raw pointers instead of Box<T> removes one layer of erasure overhead, transferring memory management of the returned class from the Rust layer to the C++ layer.
2. Extract class definitions such as Metadata and Entry from opendal.hpp into data_structure.hpp to reduce precompilation overhead from header file expansion.
3. Modified the file search approach in CMakeLists to prepare for subsequent changes. (BTW, ​we plan to​ reorganize the cpp binding directory structure, as #include "opendal/opendal.hpp" better aligns with modern C++ project conventions than #include "opendal.hpp")

Are there any user-facing changes?

nop.

cc @JackDrogon

[Xuanwo]

Inviting @silver-ymz and @xyjixyjixyji for a review.

[silver-ymz]

LGTM, Thanks.