Dual-License pkg/plugin and cmd/channels

[oxzi]

The whole Icinga Notifications project is released under some version of the GNU GPLv2. This includes pkg/plugin and cmd/channels. The plugin package is advised to be used for users to develop their own channel plugins in our documentation and the channels are especially linked as examples.

According to the GPL, software using this package must be licensed under the GPL as well. Since it is not clearly specified if the license is GPLv2 only or or later, all published user channel plugins must be licensed under the GPLv2 as well.

So I would suggest to

1. clarify the license of this project - the header in the Dockerfile implies GNU GPLv2 or later - and
2. to dual-license at least pkg/plugin also under a permissive software license, like BSD-2-Clause or MIT.

[julianbrost]

I would even consider moving it to a separate project, for two reasons:

1. Licensing is more clear: you'd import a module that's simply MIT licensed (that would probably be the most obvious choice) instead of a module with complicated licensing where you (hopefully) only use the MIT-licensed sub-packages (that might also confuse tools for license detection).
2. It would be versioned independently, so when tracking your dependency with something like dependabot, you won't get updates with every Icinga Notifications release but only if there are changes relevant to your channel implementation.