Add dynamic broker option to ot-sim app

[glattercj]

Adds an option for a key in the ot-sim phenix user app helics broker metadata that when used, will replace the minimum federate arg with --dynamic, meaning the broker will allow federates to connect dynamically even after the federation enters initialization mode. The 'dynamic' key (if set to true) will override the 'base-fed-count' key if both are included.

This feature was added in the latest HELICS release.

E.g. scenario.yml:

  - name: ot-sim
    metadata:
      helics:
        broker:
          log-level: summary
          hostname: broker|eth0
          dynamic: true

The resulting broker command for the above config would be:

helics_broker --dynamic --ipv4 --loglevel summary --logfile /var/log/helics-broker.log --autorestart &