Add rosidl_buffer and rosidl_buffer_backend for native Buffer type support

[nvcyc]

Description

This pull request adds rosidl_buffer and rosidl_buffer_backend packages - core C++ buffer types for the ROS 2 native buffer feature. This package introduces rosidl::Buffer<T>, a polymorphic container that's designed to replace std::vector<T> for uint8[] message fields (to be adopted in later pull requests; we focus on only the core buffer types in this pull request.) The native buffer type and its backend class enable vendor-specific memory buffer implementation (CUDA, ROCm, etc.) while maintaining backward compatibility for existing CPU-based std::vector<T> user code.

This pull request consists of the following key components:

• Buffer<T>: PIMPL-based container providing std::vector<T>-compatible API. All vector-compatible operations (element access, iterators, modifiers) are CPU-only and throw std::runtime_error for non-CPU backends. Backend management APIs (get_backend_type(), get_impl(), to_vector()) work for all backends.
• BufferImplBase<T>: Minimal abstract base class of buffer implementation. All backend-specific APIs are to be provided by the vendor-specific backend implementations.
• CpuBufferImpl<T>: CPU-based buffer implementation wrapping std::vector<T>.
• BufferBackend: Abstract interface for vendor-specific buffer backend implementations.

Is this user-facing behavior change?

This pull request does not change existing behavior.
When adopted in later pull requests, message types with uint8[] fields (e.g., sensor_msgs/msg/Image.data) will use rosidl::Buffer<uint8_t> instead of std::vector<uint8_t>. For CPU backends (the default), Buffer<T> is a transparent drop-in replacement.

Did you use Generative AI?

Yes. Claude (claude-4.6-opus) via Cursor was used to assist with the std::vector<T> compatibility feature in the rosidl::Buffer<T> class and generate portions of the unit tests.

Additional Information

This package/PL is part of the broader ROS2 native buffer feature introduced in this post.

[hidmic]

Amazing work @nvcyc!

[hidmic]

Looks correct.

[MiguelCompany]

LGTM