Add API to serialize to RDF formats

Merged Carlos Garnacho requested to merge wip/carlosg/serialize-api into master

Add a tracker_sparql_connection_serialize_async/finish pair of functions, that are given a RDF format from an enum, and a DESCRIBE/CONSTRUCT query, the method gives back a GInputStream that can be used to read the data in the specified RDF format.

So far "tracker3 export" has been made to use this, but it may be useful for other data migration scenarios.

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