feat(cmd/influxd): add nats-port config option to influxd server

[danxmoran]

Closes #16442

Relevant git archeology:

• nats.NewDefaultServerOptions() returns a struct with Port set to 4222
• Unit tests that used the influxd launcher sometimes failed with port conflicts because of NATS
• 047afcf added a hacky loop that would try to find an unused port closest to 4222
• fab99c9 replaced the loop with a built-in marker for random port selection

Random isn't very good for running in Docker, so we add a config parameter to support setting a specific value. I left the default as random just in case.

[davidby-influx]

Per our discussion, looks good.