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

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.

Check open pull request #935 before starting, then inspect model.bom.Bom.validate() and the validation.models package established in issue #432. Review issue #453 for the growing validation scope; done means the data model validation is encapsulated in validation.models rather than remaining on Bom.

Written by the indexing model from the issue text.