azure: Make ADU import manifest generator available independently

The current CMake configuration requires each component to have a
FetchContent declaration named the same as the component. This approach
makes it impossible to have multiple components that share a common
content declaration in DeclareSdkComponents.cmake.

Remove a call to FetchContent_MakeAvailable(${IOTSDK_FETCH_LIST}),
create a dedicated subdirectory for the script for creating ADU update
import manifest, and process `create-adu-import-manifest` like all other
components.

Signed-off-by: Filip Jagodzinski <filip.jagodzinski@arm.com>