Features for TPSA support: C++ compilation, new RawUnion type, refactor C API

[szymonlopaciuk]

Description

• Add C++17 kernel compilation support, including separate source files and configurable include/library paths.
• Support preloading shared libraries before importing compiled/prebuilt CPU kernels.
• Make CPU atomic additions compatible with C++ builds.
• Add RawUnion: a type-ID-free union for reinterpreting shared scalar storage + C API coverage.
• Extend generated C API support and tests for RawUnion fields.
• Pass container context through buffer conversion methods for nested objects.
• Refactor/document C API generation and apply formatting cleanup.

Required by: xsuite/xtrack#836.

Checklist

Mandatory:

• I have added tests to cover my changes
• All the tests are passing, including my new ones
• I described my changes in this PR description

Optional:

• The code I wrote follows good style practices (see PEP 8 and PEP 20).
• I have updated the docs in relation to my changes, if applicable
• I have tested also GPU contexts