drt: consider having long-running sessions with some simple workload

In order for us to reproduce and catch some bugs (e.g. #121844), it might require to have sessions that are running for weeks and only close when the nodes restart. We cannot really replicate such scenario neither in CI nor in roachtests, and the DRT cluster seems like a perfect fit. We should consider introducing a simple workload that would run continuously / periodically on such long-running-session.

Jira issue: CRDB-37733