Ability to get the list of resolved features in manifest mode

Hello, for a project I'm using vcpkg features in manifest mode, and I would like them to drive the actual options of the project. This is because vcpkg features determine which dependencies are retrieved, and it is important not to enable a CMake option if the dependency is not present. For now one can use `VCPKG_MANIFEST_FEATURES` to ask vcpkg some features.

Here is the feature request: it would be nice to have a CMake variable with the list of resolved features, for example `VCPKG_MANIFEST_RESOLVED_FEATURES`, so that the main CMakeLists.txt can act on it, post `project()`.

Thanks!

Start by tracing manifest-mode handling and the existing VCPKG_MANIFEST_FEATURES variable, focusing on how values become available to the main CMakeLists.txt after project(). Verify the behavior with a small manifest-mode project. Done means a documented resolved-features variable is available post-project() and reflects the features vcpkg actually selected.

Written by the indexing model from the issue text.