Download latest release of ShaderConductor automatically

[nyalloc]

I want all my sample apps to use the same set of HLSL shaders regardless of the backend being used. To facilitate this, I will use ShaderConductor to compile the HLSL shaders to the target backend's native language (SPIR-V, MSL, DXIL) and generate a header file that includes the binary.

As it currently stands, the user has to point CMake towards a local install of ShaderCompiler using SHADER_CONDUCTOR_INSTALL_DIR. This works just fine, but it's a bit annoying from a user perspective. It would be nice if we could automatically download a ShaderConductor release if SHADER_CONDUCTOR_INSTALL_DIR is not set by the user. There are a lot of good examples on how to do this in the wild.