Rudder plugins

We need to be able to build licenced file from the top level make, and be able to build the dependency library if needed.

Ideally, we want to be able to do:

# from .../rudder-plugins:

# build plugins-common and each .rpkg plugin
make

# build only a given rpkg:
make datasources

# build plugins-common, plugins-common-private and each .rpkg licensed plugin
# using the given licensed file and (the same) public key
make licensed SIGNED_LICENSE_PATH="./license.sign"  PUBLIC_KEY_PATH=/path/to/demo/public/key/demo.pub

# only make datasources licensed plugin:
make datasources-licensed SIGNED_LICENSE_PATH="./license.sign"  PUBLIC_KEY_PATH=/path/to/demo/public/key/demo.pub