Detect parent project VCPKG

[dbs4261]

Check for existing VCPKG usage in a consuming project and dispatch to that implementation. This could be done using the Z_VCPKG_ROOT_DIR variable defined in the toolchain file vcpkg/scripts/buildsystems/vcpkg.cmake. This should also look up the necessary parameters for VCPKG setup, for example using the existing commit and not creating a new directory tree. This is required if the consuming package is in manifest mode:

error: In manifest mode, `vcpkg install` does not support individual package arguments.
To install additional packages, edit vcpkg.json and then run `vcpkg install` without any package arguments.