[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.