qsb cannot find spirv-opt made available through vcpkg

When calling qt_add_shaders with the OPTIMIZED setting, qsb will be requested to perform shader optimizations using spirv-opt.

The issue is that qsb calls spirv-opt just by its name, which only works if spirv-opt is in PATH or in the same directory as qsb.

This breaks when spirv-opt is made available through vcpkg, as the executable's directory is not added to PATH.
Instead, the expected workflow is to first request its absolute path using the find_program CMake function and then use it from this path.

Adding a new option to qsb that allows users to pass an explicit path for spirv-opt (and exposing this option in qt_add_shaders) should fix the issue.