Add new step pacdiff

[mhaessig]

This PR adds a step to execute pacdiff. It is run with sudo since it needs to edit files in /etc and it can be disabled.

Checks:

• [x] The code compiles (cargo build)
• [x] The code passes rustfmt (cargo fmt)
• [x] The code passes clippy (cargo clippy)
• [x] The code passes tests (cargo test)
• [x] Optional: I have tested the code myself
  • [x] I also tested that Topgrade skips the step where needed

This PR closes #794