[datafeed] allow to use cwclientlib for datafeed's queries (closes #5456849) HTTP based URLs will be handled by a CWProxy if: - cwclientlib is available and - the URL matches a cwclientlibrc config entry Otherwise, fallback to previous implementation. See https://www.cubicweb.org/project/cwclientlib :Warning: This comes with an API modification of DataFeedParser.retrieve_url: it used to accept data and headers arguments to build GET or POST (if data was given) queries, but this was not used by any known code and imply more complicated code.

[cwxml parser] refactor url handling code to use urlparse and urlencode (related to #5456849)

add use_extid_as_cwuri ext entity transform, that will be often necessary and not so easy to write at once