feat: read License File tag from environment

[a1lu]

The package metadata can contain a license_file entry that is not handled currently.

[thbeu]

What do you expect exactly to read from the license_file? Do you want to have it set as the text attribute of the license data?

[a1lu]

Yes, we could do it like
https://github.com/CycloneDX/cyclonedx-python/blob/master/cyclonedx_py/parser/environment.py#L76
and just read the file into a License.

[jkowalleck]

duplicates #570