add gstreamer-allocators

Added a rust wrapper for gstreamer-allocators-1.0 generated from the GstAllocators-1.0.gir file. So far i tested the DmaBufAllocator, which seems to work fine. I used an external generated fd that is passed to the DmaBufAllocators and used in a Pipeline with an AppSrc. I tried with direct presentation using waylandsink, autovideosink, 'xvimagesink' and encoding using vaapipostproc and vaapih264enc. The fd was generated by exporting a dma buf fd from libva. I have also tried to use a linear buffer created with gbm, which also worked.