[repository] #1460066: backport datafeed cube as cubicweb source * add some attributes to CWSource to handle this kind of source (not natural to put everything in 'config' string). Adding a CWSource subclass has been attempted then rollbacked because it adds pain to handle multi-sources planning and it introduce an ambiguity on a generic relation (cw_source), which may be a penalty in multiple case * data feed sources are a new kind of source, namely 'copy based', which have no effect on the query planner * a data feed source is associated to a list of url and a parser (appobjects in the 'parsers' registry * entities imported by a data feed have cwuri set to their url on the distant site, their cw_source relation point to the data feed source, though their source stored in the entities table (returned by cw_metainformation) is their physical source, hence 'system'

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