Update UrlSynchronization to limit amount of downloads (#2586) (#2933)

Limits the number of URL synchronization downloads by time stamping files for X seconds fixes (#2586). 
Also, show the amount of downloaded data as soon as the download is started fix (#2460)

* Adds time validity to UrlSynchronized files (#2586)

Updated affected asset files. Satellite asset files lasts for 24h by default now. Other assets are updated to work as before. Obs timestamp is computed using UTC time.

* Code cleanup

* Update httpsynchronization.cpp

* Applied suggestions from code review

* Update multi-line comment to `//`

* Remove accidental commas in .asset files

Co-authored-by: Emma Broman <emma.broman@liu.se>

* Fixed PR comments + bug in .asset file

* Refactor code

* Reports downloaded data as early as possible, fixes (#2460)

* More files

---------

Co-authored-by: Alexander Bock <alexander.bock@liu.se>
Co-authored-by: Emma Broman <emma.broman@liu.se>

include/openspace/util/spicemanager.h

@@ -1085,6 +1085,9 @@ private:
     glm::dmat3 getEstimatedTransformMatrix(const std::string& fromFrame,
         const std::string& toFrame, double time) const;
 
+    /// Loads pre defined leap seconds time kernel (naif00012.tls).
+    void loadLeapSecondsSpiceKernel();
+
     /// A list of all loaded kernels
     std::vector<KernelInformation> _loadedKernels;