Add inadyn as an alternative to ddclient

[sdf_iain]

Inadyn, https://github.com/troglobit/inadyn, seems like a good third option for a Dynamic DNS client (in addition to ddclient and dyndns), so I thought I'd try my hand at building a OPNsense package for it.

As such I have thrown together a plug-in for it, but I have a few questions before I open a pull request:

• If the client is a dependency in the Makefile, will it be built in the OPNsense package repo?
• How do I make this a service that can be managed from the gui (start, stop, restart from any of the settings pages)? I can see Unbound is set up this way, but I can't see where that is.

here is the repo with the code:
https://github.com/iain-henderson/plugins/tree/master/dns/inadyn