[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.