provide a mechanism to add a license header to generated files

[smola]

In order to comply with licensing requirements, some projects require that every file, included autogenerated ones, start with a predefined copyright header.

It would be nice if proteus provided a way of specifying that header. Maybe with a CLI option --header <path-to-header-file>.

My initial idea was having a flag --include-header that takes the license header from source files, if any. But as @Serabe pointed out, a proto can be generated from many files, so making it automagic is problematic.

[Serabe]

As a general note, the k8s tool had an option like this and it worked giving the path to a text file.

This can be a solution to this problem, though more planning is needed.