files: clarify transfer object/factory responsabilities and calling order

[ppanero]

The Transfer object is called from within the service components:

FileService -> {Metadata/Content}Component -> Transfer.

However, the Transfer objects sometimes need the service (id) and uow, for example, to register the file fetching task (FetchTransfer).

The calling flow (see above) is not very transparent, since we are calling a class while just doing parameter forwarding. Can this be split/encapsulated better?

Would it make sense to have a TransferComponent? This would already have a service and uow and could perform some checks and only register the tasks then. On the other hand, it would expose the TransferType (or how to check if not local?)

[max-moser]

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