Add support for rosidl::Buffer type serialization

[nvcyc]

Description

This pull request adds support for rosidl::Buffer-aware serializaion in rosidl_typesupport_fastrtps_cpp.
operator<< / operator>> for rosidl::Buffer are provided to offer backward-compatibility in the plain operator path which ensures non-CPU buffers are converted to std::vector before serialization.
Backend-based endpoint-aware serialization/deserialization functions (serialize_buffer_with_endpoint()) are added to support backend-backed buffer serialization/deserialization path. To serialize a non-CPU buffer, a backend callback (create_descriptor_with_endpoint()) is invoked to export the buffer as a backend-defined buffer descriptor message before serialization.

A CPU-based buffer (std::vector) is serialized exactly the same as the original std::vector<uint8_t> bytes (uint32 length + raw data) to preserve compatibility with existing traffic and rosbag2.
For the serialized bytes of a non-CPU buffer descriptor, it starts with kBufferDescriptorMarker = 0xFFFFFFFF followed by the backend_type string, then the serialized descriptor. Deserialization peeks the first uint32 to check if kBufferDescriptorMarker is present for the descriptor path; any other value is treated as a CPU-based buffer (std::vector) sequence length.

This pull request consists of the following key changes:

• buffer_serialization.hpp: New header with template helpers for rosidl::Buffer serialization and global registries for backend descriptor operations (BufferDescriptorOps) and FastCDR serializers (DescriptorSerializers).
• Type support callbacks: Extend the callbacks struct with a has_buffer_fields flag and endpoint-aware cdr_serialize_with_endpoint / cdr_deserialize_with_endpoint function pointers, enabling per-endpoint serialization decisions.
• register_buffer_descriptor.hpp: register_buffer_descriptor<DescriptorMsgT>(backend_name) lets backends register FastCDR serializers using existing rosidl-generated type support. Backends are expected to call this in their constructor.
• Code generation templates: Updated to detect Buffer fields and emit endpoint-aware serialization code.

Is this user-facing behavior change?

This pull request does not change existing behavior. The serialization of uint8[] fields is wire-compatible with the existing std::vector<uint8_t> format. The new descriptor path is only activated when users publish messages with non-CPU buffers.

Did you use Generative AI?

Yes. Claude (claude-4.6-opus) via Cursor was used to assist with creating an initial prototype version of the changes contained in this PR.

Additional Information

This PR is part of the broader ROS 2 native buffer feature introduced in this post. It depends on PR #491 (rosidl_buffer + rosidl_buffer_backend) and can be reviewed in parallel with PR #492 (C++ rosidl changes).

[hidmic]

First pass. This code needs a lot of eyes. EmPy templates are always incredibly tough to read.

[hidmic]

@nvcyc hmm, this is.. unusual. Should we be passing the impl by reference to create_descriptor_with_endpoint()? Why is the buffer passed by const ref if we need to mutate it?

[hidmic]

@nvcyc meta: hmm, I'm confused. If the descriptor is serialized, won't it usually be much smaller than the buffer? Isn't the descriptor the metadata itself?

Either way, I'm fine with a metadata overhead bound but I do think it should be a public constant that backends should use to enforce the bound on descriptor serialization.

[richiware]

The last serialized element was a string. If T is larger than 1 byte, the first element in the buffer must be properly aligned.