Uniformization of get_modified_files

[mgoerens]

We're using various methods to collect the list of modified files:

• jitterbit/get-changed-files action: this action has seen its last commit 4+ years ago (May 2021)

• get_modified_files() in prartifact.py

• _get_modified_files private method of the Submission class

• Custom github script using github.rest.pulls.listFiles: In this specific case, the goal is also to determine if the file(s) is added or modified by the PR.

We should determine a unique way to perform this action