Resolve "Refactor ObserverService"

Description

Refactor ObserverService to make it better understandable. The goal is also to make him fetch clean code design principle

This MR is the final container for a set of smaller atomic MR which would allow an better review:

• !156 (merged)
• !155 (merged)
• !153 (merged)
• !152 (merged)
• !151 (merged)

All of them have already been reviewed and approuved

Issue(s)

https://gitlab.e.foundation/e/os/eDrive/-/issues/144 https://gitlab.e.foundation/e/os/backlog/-/issues/657

Check list

• Self review
• Test procedure explained
• Tested on fresh install
• Tested applied as an update
• License
• Internal documentation
• User documentation

Closes #144