skywire cli subcommand + api to disable dmsg connection at runtime (for testing)

[0pcom]

In order to test the service which will restart the visor on loss of connection to dmsg, it's necessary to be able to explicitly disable the dmsghttp logserver as the /health endpoint is the one which will be checked by the service.

If not like that, then it must be possible by some other means to break the connection to dmsg in order to test this service or a future fix in the codebase reconnect logic.

I think it will not work to disable the connection to dmsg at startup as the visor requires this to even start correctly. But if possible to disable dmsg during runtime via skywire cli subcommand and RPC / API; that will work. as well as disabling the logserver.

The systemd service which checks if the visor is connected to dmsg will use dmsgcurl

dmsgcurl currently does not work with dmsghttp-config.json and will not be useful for chinese users (and will likely restart the visor inappropriately) until that is implemented.

[0pcom]

as noted in #1804 the dmsghttp log server may become inaccessible even though the visor apparently maintains a connection to dmsg.