feat: add `bom.definitions`

[hakandilek]

This might also be considered as a part of #696 since the bom.definitions is introduced in v1.6.

We're currently using bom.definitions to indicate that the given SBOM aligns with certain standards. It'd be nice to have it supported in the library.

[jkowalleck]

@hakandilek ,

You know your use cases best, and you could provide examples and tests.
Would you be interested in contributing this feature?