ENH: Add SPDX 2.3 SBOM generation at configure time

Implement build-time generation of a Software Bill of Materials (SBOM) in
SPDX 2.3 JSON format. The SBOM documents ITK and all enabled third-party
dependencies with their names, versions, SPDX license identifiers, and
dependency relationships.

- Add CMake/ITKSBOMGeneration.cmake with SBOM generation logic
- Add ITK_GENERATE_SBOM option (default ON) to CMakeLists.txt
- Generate sbom.spdx.json in the build directory at configure time
- Install sbom.spdx.json alongside LICENSE, NOTICE, and README.md
- Support FFTW as optional GPL dependency in SBOM
- Provide itk_sbom_register_package() for remote modules to extend SBOM

Co-authored-by: dzenanz <1792121+dzenanz@users.noreply.github.com>

Author: GitHub Copilot