d/survive: Add libsurvive with positional tracking as submodule

.gitmodules

CMakeLists.txt

README.md

src/CMakeLists.txt

src/external/libsurvive

src/xrt/drivers/CMakeLists.txt

src/xrt/drivers/survive/survive_device.c

src/xrt/drivers/survive/survive_device.h

src/xrt/drivers/survive/survive_interface.h

src/xrt/drivers/survive/survive_prober.c

src/xrt/targets/openxr/CMakeLists.txt

src/xrt/targets/openxr/target.c