feat: make add_dataclass_options public, separate field extraction into public helper function

[johanneskoester]

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This PR adds two things:

1. add_dataclass_options becomes a public function, which fixes #58
2. I have separated the logic that infers args and kwargs for add_argument into a separate public helper function. This allows to further customize the added argument if needed. We use this in https://github.com/snakemake/snakemake-interface-executor-plugins to add additional arguments that are needed for e.g. utilizing the configargparse package in combination with argparse_dataclass.

I apologize for the mixed up diff. In fact, field_to_argument_args is just an exact copy of the inner loop that was formerly implemented in add_dataclass_options. Also note that this PR does not introduce a breaking change, it just adds to the public API the two functions.

[mivade]

Looks like there is some typing issue that's failing on Python 3.8 but I haven't spotted what exactly it is yet.

[martinResearch]

Could you please add a test like I did in my draft PR? https://github.com/mivade/argparse_dataclass/pull/60 thanks!

[vsoch]

Any progress here?