refactor: reuse bomref-snippet

via https://github.com/CycloneDX/cyclonedx-python-lib/pull/701/files#diff-c90c501398d3d23a7a10846ec3ff03fb092a7c1a8f2fc90f79a29782892efcea

the duplicated code for bom-ref initializations in classes was extracted.

after the #701 was moved, the goal shall be:
find all these code duplications, and use the newly extracted function instead.