GDPR export should offer more granularity for what to export

[Niko-O]

Is your feature request related to a problem?

Same problem as #28487

Describe the solution you'd like

I would like to see an addition to the export dialog where I can select for each group individually, whether I want to export all messages in that group, only my messages, or not export it at all, (or my messages and context, if #28487 is implemented).

Describe alternatives you've considered

Only exporting my own messages is not a valid solution because I want all messages (mine and others) for some groups.

Additional context

No response