roachtest: create a workload that tests latencies of repeated connection attempts

[rafiss]

The bug in #58671 caused the latency of establishing a secure connection to a multi-region cluster for a non-root user to dramatically increase. None of the roachtest workloads really test this, since they all use a connection pool where the connections are created once in the beginning, then reused for the rest of the test.

Create a test which is specifically for testing connection latency in a CockroachDB cluster. The test should report its results in roachperf.crdb.dev/ so that we can track performance changes over time.

Let's also just spot-check the performance of 21.1 compared to the latest 20.2 release.