refactor(vex): improve SBOM reference handling with project standards

[knqyf263]

Overview

This PR refactors the SBOM reference handling code to better align with project conventions.

Description

The changes focus on improving the code structure and readability in the VEX SBOM reference implementation:

• Added context-based logging for improved traceability and debugging
• Utilized lo.FilterMap for more concise and functional URL parsing
• Simplified error handling and logging patterns
• Improved code structure with cleaner function signatures and return patterns
• Enhanced test readability by using table-driven tests with clear test cases
• Refactored external VEX document retrieval with table-driven tests for better test coverage and maintainability

These changes maintain the same functionality while making the code more maintainable and consistent with the project's coding standards.

PRs

• feat(cyclonedx): Add initial support for loading external VEX files from SBOM references #8254

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[DmitriyLewen]

LGTM