Support cloning problemset.

[cubercsl]

In some cases, two contests (on the same domjudge instance) share the same problemset (and balloon color). Currently, there's no automated way to quickly clone these configurations. This is because importing problems.yaml attempts to create new problems using the ids specified in the file and then associate them with the contest, but these problems may already exist.

Current workaround is manually create a new contest, then add exist problems and set colors one by one.

Context: https://domjudge-org.slack.com/archives/CHHURFUM7/p1759656426669289?thread_ts=1759656426.669289&cid=CHHURFUM7

[vmcj]

Just for other coming to this issue, there is the endpoint: /api/v4/contests/{cid}/problems/{id} (Link an existing problem to this contest.)

I think you can use that one to link the problem to the other contest. It wouldn't be as nice as uploading the problemset.yaml twice.