qt_generate_deploy_app_script: Add a way to specify an additional CMake script file

qt_generate_deploy_app_script generates a CMake script that calls qt_deploy_runtime_dependencies.

For setups that need customization, we currently have to generate the script like this:

set(deploy_script "${CMAKE_CURRENT_BINARY_DIR}/deploy_MyApp.cmake")
file(GENERATE OUTPUT ${deploy_script} CONTENT "
include(\"${QT_DEPLOY_SUPPORT}\")

qt_deploy_runtime_dependencies(
    EXECUTABLE \"\${QT_DEPLOY_BIN_DIR}/$<TARGET_FILE_NAME:MyApp>\"
    GENERATE_QT_CONF
)")

There's a lot of escaping going on. That doesn't help with readability.
It would be nice if the deployment code could live in a separate .cmake file.

Note that we cannot use file(GENERATE OUTPUT ... INPUT ...), because there, we don't have variable expansions, and we need to expand QT_DEPLOY_SUPPORT for the first include statement.

It would be nice to have a convenient way to specify the deployment code in a separate file. Maybe like this:

  qt_generate_deploy_app_script(
      TARGET MyApp
      FILENAME_VARIABLE deploy_script
      INPUT ${CMAKE_CURRENT_SOURCE_DIR}/deploy_myapp.cmake
      NO_UNSUPPORTED_PLATFORM_ERROR
  )