Added interface for encoder.

CMakeLists.txt

src/encoder/encoder.cpp

src/encoder/encoder.hpp

src/encoder/nvidia_encoder.cpp

src/encoder/nvidia_encoder.hpp

src/encoder/vulkan_encoder.cpp

src/encoder/vulkan_encoder.hpp

src/headset/remote_headset.cpp

src/headset/remote_headset.hpp

src/stream/transport.hpp

src/utility/command_parser.cpp

src/utility/command_parser.hpp

src/utility/extern_fence.cpp

src/utility/extern_fence.hpp

src/vr_application.cpp

src/vr_application.hpp