[WIP] New GitExtraGenerationBuilder builder

[lneves12]

New builder to allow the inclusion of generated files that are not tracked by git on the final tar.gz

As the tito solution relies on "git archive" this was the only undisruptive solution I could find without keeping the generated files on git.
Basically we are generating a new git reference on the top of the tag we are building and use it for the git archive, and afterwards we discard everything.

Each tito module can have 2 new files to configure this task:

setup.sh (example)
yarn install

tito.gitgeneratedbuilder.include (example)
susemanager-nodejs-sdk-devel.tar.gz

[WIP]
This still need some more tests and a better error handling, in case some command fail there is the risk that git gets on an inconsistent state.

But before having a bullet proof solution would be nice to have more opinions about the direction of this solution.

@mcalmer

[lneves12]

It seems TITO allows to define custom code on the project level so I moved it there. We need to decide if we want to include this on TITO itself or keep it inside the project.

For me it would make sense to keep it on the project level