Introduce Qt CMake API policies to control public CMake API behavior similar to CMake's own policies

In the future we might need to change the behavior of public CMake API functions.
To do that in a backward compatible manner, we could implement Qt's own CMake API policies modelled similar to CMake's policies.

The first case where this would be relevant is to change the behavior of qt_import_qml_plugins as discussed at https://codereview.qt-project.org/c/qt/qtdeclarative/+/369066/1/src/qml/doc/src/cmake/qt_import_qml_plugins.qdoc#48

A potential new command could be

qt_set_policy(<policyID> REQUIRES <minPolicyVersion> SAFE_TO <maxPolicyVersion>)

We might also want or not want to provide subdirectory and target specific commands.