feat: release automation

[theseion]

Fixes #140

[theseion]

@fzipi This is a draft for release automation. Currently implemnted:

1. create new branch for release
2. update version and copyright
3. create commit
4. push branch
5. create PR

The remote operations are not covered by unit tests but I've tested them a bit manually. I'm sure that stuff will fail but my idea was to use this version to create the release PR for the upcoming release and fix issues as we see them.

[fzipi]

I see a mix of exec.Command git and also git v5 library usage. Can we consolidate in one of the methods?

[theseion]

I see a mix of exec.Command git and also git v5 library usage. Can we consolidate in one of the methods?

That's on purpose. The CLI takes care of all the complicated stuff like commit signing etc., while accessing information is simpler through go-git, as I don't have to parse the information (well, apart from figuring out the remote, which I should probably use go-git for as well).