McuSupport: integrate vendor "MCU SDKs" into Qt for MCUs kits

In order to access target specific APIs/services, one needs a vendor specific SDKs. In Qt for MCUs, these SDKs get integrated to the build system by a couple of environment variables.

Some examples: EVKB_IMXRT1050_SDK_PATH, STM32Cube_FW_H7_SDK_PATH, STM32_CUBE_L4_INSTALL_PATH, etc.

See: https://doc.qt.io/QtForMCUs/qtul-getting-started-on-stm.html#board-specific-instructions

Task: The Qt for MCUs plugin in Qt Creator needs to allow the user to specify the path of the target/board specific SDK and add that information as build environment variable to the Kit. Since these vendor SDKs will be shipped via the Qt SDK installer (UL-2531), and their paths will be registered in Qt Creator (UL-2557), the paths will become a required for the Kit creation.