Improve contrib experience with magefiles

[khaosdoctor]

Today there are a lot of makefiles to run scripts in the project, they're fine but take a while. The docker-push-all task runs all the builds sequentially while they could be running in parallel, this is just an example.

I propose to replace makefiles for magefiles as @arschles pointed out in one of our discussions and port the scripts to these files. This will allow us to have a better handling of files and allow us to do more complex tasks in the dev environment.

Plus, to improve contribex we can also version and ship the mage binary along with the repository to make the usage easier.

[khaosdoctor]

Doing it on #56