Rudder plugins

We want to be able to make standard protected binaries and give the license / key separatly. We used to embed both to make the life of users simpler, but that forbid (among other things) some important maintenance tasks like:

- changing the public key if needed,
- easely updating the license information,
- having verifiable signatures common for users of the same binary (each binary is unique).

And of course, it simplify the distribution of binaries.

So, we need the common makefile infrastructure so that make ${plugin}-licensed build a generic licensed plugin which will look:

- for the license in path: /opt/rudder/etc/plugins/licenses/${pluginId}.license
- for the key path (the key is the same for all plugin of a given user): opt/rudder/etc/plugins/licenses/license.key