CMake: Move cellular stub headers to the cellular/tests/UNITTESTS/double dir

Previously all the stubs headers under mbed-stubs-headers,
so this PR move all cellular stubs headers under
cellular/tests/UNITTESTS/double directory with new
mbed-stubs-cellular-headers library to make cellular stubs to be self-contained

Author: rajkan01

connectivity/cellular/tests/UNITTESTS/CMakeLists.txt

@@ -1,4 +1,5 @@
 # Copyright (c) 2021 ARM Limited. All rights reserved.
 # SPDX-License-Identifier: Apache-2.0
 
+add_subdirectory(doubles)
 add_subdirectory(framework)

UNITTESTS/stubs/ATHandler_stub.h renamed to connectivity/cellular/tests/UNITTESTS/doubles/ATHandler_stub.h

@@ -0,0 +1,9 @@
+# Copyright (c) 2021 ARM Limited. All rights reserved.
+# SPDX-License-Identifier: Apache-2.0
+
+add_library(mbed-stubs-cellular-headers INTERFACE)
+
+target_include_directories(mbed-stubs-cellular-headers
+    INTERFACE
+        . 
+)