Add task.ext.suffix for CSVTK_CONCAT

[LeonHafner]

This PR adds support for specifying a custom suffix, so pipeline maintainers can work with files beyond just .csv and .tsv.

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Closes #XXX

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[mahesh-panchal]

In my opinion the file ending should only be an input:. I don't see the case where a user needs to change it, as it could lead to downstream instability.

[LeonHafner]

I’m fine with any solution that lets me use it for files other than CSV and TSV.