Add get-changed-files action

[IlyaKhD]

Practical Value

Removes duplication in actions and makes internal logic for getting changed files more reliable

Before

• 2 implementations for same purpose of getting changed files (the 2nd one is in the monorepo: get-changed-files/action.yml)
• the implementation that is used in this repo is based on operating with git commands by determininh head and base commits. This way has its pitfall - all changes from the commits that are in the master branch but not in the PR branch (when it lags behind the master branch) are considered as changed files, which is wrong

After

• 1 implementation is used in all actions (the one from the monorepo will be removed)
• thisi mplementation is based on GH API and is much more reliable