[dataimport] introduce the importer and extentity classes This introduces the ``ExtEntity`` class which is a transitional state between data at external source and the actual CubicWeb entities. ``ExtEntitiesImporter`` is then in charge to turn a bunch of ext entities into CW entities in repository, using a given store. This changeset also introduces ``SimpleImportLog`` and ``HTMLImportLog`` which implement the CW DataImportLog interface in order to show log messages in UI using simple text and HTML formats respectively, instead of storing these messages in database. Both have mostly been backported from cubes.skos.dataimport. Closes #5414753.

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TACHES

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-- écrire objet stocke/manipule les données

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-- extraire tests de chaîne de caractère

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* utiliser sqlite 

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* écrire fonction prend chemin en argument et renvoie contenu

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* extraire tests (chaîne de caractère) de spec 

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* utiliser yams pour schéma