move `model.bom.Bom.validate()` to `validation.models`

[jkowalleck]

now that a package validation was established via https://github.com/CycloneDX/cyclonedx-python-lib/issues/432, t should be possible to move the data model validation there, too.\

data model validation is getting more and more complex https://github.com/CycloneDX/cyclonedx-python-lib/issues/453 and should be encapsulated and could be devided.