[#76] Fixed problems with custom shader stages

include/vkcv/DescriptorConfig.hpp

include/vkcv/ShaderProgram.hpp

include/vkcv/ShaderStage.hpp

projects/first_mesh/src/main.cpp

src/vkcv/DescriptorConfig.cpp

src/vkcv/DescriptorManager.cpp

src/vkcv/PipelineManager.cpp

src/vkcv/PipelineManager.hpp

src/vkcv/ShaderProgram.cpp