libtracker-sparql: Add tracker_sparql_connection_bus_new_async/finish

The bus connection started creating a dbus proxy that we did not need to autostart, nor read properties from, so we could initially afford to not have an asynchronous version.

But later we added more things on it, like session registration in the portal, and pings to check for direct access permissions within a possible sandbox.

It now makes sense to have that async variant, so add one.

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Note: At this point this needs approvals from the release team.