Allow addons via python.

[bartossh]

The facade interface for validation will allow validation to be implemented in Go. But this will be limited to adapt or use C9s by non-Go programmers. Python language is easy to learn and most programmers may use it efficiently, so adding the possibility to interact with Python scripts is important for Computantis to be adaptive.

[bartossh]

Small update. After giving it a second thought I think C9s shall talk to Addons via GRPC protocol. It will allow us to not embed interpreters or run cmd on the system which is unacceptable from a security perspective.