[D8] Handle file transfer

About file transfer, the file entity is created but not the physical file.

Case of public files:

Case 1: the file entity (same UUID) does not exist and there is no physical file matching the URI

Create the entity and transfer the file.
On the next synchronisation, we will be in case 4.

Case 2: the file entity (same UUID) exists and there is no physical file matching the URI

Update the entity and transfer the file.
On the next synchronisation, we will be in case 4.

Case 3: the file entity (same UUID) does not exist and there is a physical file matching the URI from the source file entity (public://test.txt)

Transfer and rename the file (public://test_0.txt), create the entity with the URI of the renamed file (public://test_0.txt).
On the next synchronisation, we will be in case 4.

Case 4: the file entity (same UUID) exists and there is a physical file matching the URI of the existing file entity (public://test_0.txt)

Update the entity and overwrite the physical file (or transfer the file only if necessary)
On the next synchronisation, we will be in case 4.

Case of private files:

Handle an authentication using webservices and then fallback in the logic for public files.