ci: add Sourcery GitHub action

[ruancomelli]

Overview

Define a new re-usable GitHub Action for Sourcery.

Follow-up work

• add this to our docs;
• publish this action to the GitHub Marketplace.

Question for the reviewers

I am not 100% sure about making the input parameter base_ref public and "stable", since it is likely to change in the near future once Sourcery can be applied to changed code only. What do you think?