Allow running performance test server and simulations separately

[kciesielski]

This PR adds some flexibility in the perfTests module. So far it was only possible to run a "test suite" with specified servers and simulations. Server list will now become optional, which allows running a profiled server separately:

perfTests/runMain sttp.tapir.perf.apis.ServerRunner netty.cats.TapirMulti
// in a separate sbt session:
perfTest/Test/runMain sttp.tapir.perf.PerfTestSuiteRunner -m PostBytes,PostLongBytes -d 25

The perfTests/runMain task forks a JVM with

javaOptions += "-XX:StartFlightRecording=filename=recording.jfr,dumponexit=true,duration=120s"