Enable user specified tags to be added to darktable image as part of export process

[fjb2020]

Is your feature request related to a problem? Please describe.
I like to keep track of which images I have exported to various external services, and use tags which I manually add to images in darktable after I have exported them.

Describe the solution you'd like
It would be really helpful if an additional text field, say 'Tags' added to the export dialogs into which I could enter tags - that would be attached to any images if they are exported via this export service. These tags would be stored in any presets created for ease of retrieval and reuse. The tags would not need to be included in the actual exported image, just to the 'source' image in the darktable database.

Alternatives
For disk based exports I (and others) have written custom lua scripts which can incorporate this functionality. However, one of the export services I use regularly, Piwigo, does not lend itself easily to this approach given the complexity of interfacing to Piwigo properly - therefore having this feature built into the core export functionality would be really helpful.

Additional context
This is a screenshot of the ExportToPublish script by Christian Birzer showing a tags text field as described above - if something similar could be added to the inbuilt export funcationality (esp. Piwigo) that would be great.

[github-actions]

This issue has been marked as stale due to inactivity for the last 60 days. It will be automatically closed in 300 days if no update occurs. Please check if the master branch has fixed it and report again or close the issue.

[ralfbrown]

Depending on which Exif/Xmp/IPTC field you want your tags inserted in, if you don't want to preserve other keyword tags, a fixed tag or tags can be added by defining an appropriate redefinition in the export module's preferences. Presumably you'd want to add to Xmp.lr.HierarchicalSubject, Iptc.Application2.Subject, or Iptc.Application2.Keywords.