Add creation date as parameter, allow both dates and offsets as values, proof of concept for #142

[bkleinen]

Here's my proof of concept for issue #142, introducing both a creation date parameter "created_at" and the option (just for this parameter for now) to have both offsets and dates as values.

This implementation is a proof of concept, I've implemented it without changing any other methods.

If you like the idea to have both kinds of values for all date parameter, some refactoring should be done.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[mikez]

If you want to add creation_date as a parameter, I'd follow the "start_date" and "stop_date" pattern... and add a parameter called "creation_date" to things.tasks.