Implement new classes to handle listing of existing content

Description

Extract and improve code from ListFileRemoteOperation & ObserverService into dedicated classes:

• AbstractFileLister: which contains the workflow and algorithm for listing file
• LocalFileLister: which contains implementations of AbstractFileLister for Listing files on the device
• RemoteFileLister: which contains implementations of AbstractFileLister for listing files from the cloud

This merge request contains also, tests classes to assert behaviour of FileListerClasses:

• LocalFileListerTest
• RemoteFileListerTest

Technical details

The tests uses mockito a lot.

Tests

• https://gitlab.e.foundation/e/os/eDrive/-/quality/test_cases/150
• https://gitlab.e.foundation/e/os/eDrive/-/quality/test_cases/146
• https://gitlab.e.foundation/e/os/eDrive/-/quality/test_cases/145 : failed first time due to network issue, then file ignored. But it's not related to this MR.

Issues

backlog#762

10 commandments of code reviews

👪 ❤ https://gitlab.e.foundation/internal/wiki/-/wikis/development/code-review