Move cellular stubs to Connectivity/cellular directory

[rajkan01]

Description of defect

Currently, all connectivity AT, cellular, LoRa.. stubs cpp files are grouped under mbed-stubs-connectivity as a single library, so split these single library and create a new mbed-stubs-cellular, mbed-stubs-cellular-headers library for cellular stubs under connectivity/cellular directory, so the stubs live with the associated component

Reference PR: #14773

Target(s) affected by this defect ?

N/A

Toolchain(s) (name and version) displaying this defect ?

N/A

What version of Mbed-os are you using (tag or sha) ?

Latest

What version(s) of tools are you using. List all that apply (E.g. mbed-cli)

N/A

How is this defect reproduced ?

N/A