cmake: Rename project to open-iot-sdk

The old project name `IoTSDK` does not conform to our convention of
naming CMake targets with lowercase alphanumericals and dashes
(`[a-z0-9\-]+`). Use `open-iot-sdk` instead.

This is *not* a breaking change for existing applications that depend
on the SDK, because
* applications only link *libraries* fetched by the SDK, not the SDK
itself which is a project
* in the fetched path to the SDK, `<NAME>_SOURCE_DIR`, `<NAME>` is
whatever name the application passes to `FetchContent_Declare()` instead
of the SDK's own declared name (although it is strongly recommended to
use the same name for FetchContent, git repository and CMake project)

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>