cmake: allow client apps to load static plug-ins

Based in part on Kyle Edwards's implementation :
https://codereview.qt-project.org/c/qt/qtbase/+/243731

Example :
```
cmake_minimum_required(VERSION 3.15)
project(foo)

add_executable(foo main.cpp)

find_package(ICU COMPONENTS i18n uc data REQUIRED)
find_package(Qt6 COMPONENTS Core Gui REQUIRED)
target_link_libraries(foo Qt6::Core Qt6::Gui)

qt_import_plugins(foo
    INCLUDE
        Qt6::qxcb
    EXCLUDE
        Qt6::qgtk3 Qt6::qeglfs-kms-integration Qt6::qjpeg
)
```

Change-Id: If7736c42f669f7d7f43052cae59c28fc7fcb4156
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

0 files changed, 0 insertions, 0 deletions