make abstract distinct from description and summary

[broeder-j]

Is your feature request related to a problem? Please describe.
Currently, somesy.toml allows to set a description key value, this will get put into the CITATION.cff as abstract and into codemeta and pyproject.toml as description.

Python project metadata description corresponds to package metadata summary, which is limited on pypi to be < 512 chars.
(https://packaging.python.org/en/latest/specifications/declaring-project-metadata/#description)

Therefore, if one wants to put a real abstract into the CITATION.cff, it should not be synced with the pyproject.toml description, otherwise the package is not uploadable to pypi.

Describe the solution you'd like
I suggest to allow for an optional 'abstract' keyword in somesy.toml, which defaults to a given value under the description key if provided.