Individual libraries' `target_h` stub headers have now all been moved
from `mbed-headers-base` to `mbed-headers-<library>`.

Note: Even though headers previously in `target_h` are technically
stubs/fakes too, they are used by not only unit tests but also regular
libraries when compiled for unit tests, because no target-specific HAL
implementation exists in this case. In order for regular library
sources to pick up `target_h` headers, those headers must

* have the same names as regular headers
* appear first in include paths

This is why those headers are part of `mbed-headers-<library>` and not
`mbed-stubs-<library>`. Before this refactoring, `mbed-headers-base`
was the first in unit tests' include paths.

Update AT cellular unit tests CMake depends on a mock, which is required
for the test instead of depending on the group of mbed-stubs,
mbed-headers, mbed_stub_headers, libraries.

- Previously, all netsocket headers made as a part of the
mbed-headers-connectivity library. To make it easier to separate all
the netsocket headers into the separate mbed-headers-netsocket library
and keep them, as part of netsocket stubs CMake. This makes the
netsocket stubs more self-contained and improves the composition of the library.
- Update cellular, netsocket unit tests to link only with a mbed-headers-netsocket
library that it depends on instead of link with mbed-headers
which is a group of headers libraries.

Update netsocket and cellular unit tests to link with a mbed-stubs-netsocket
library that it depends on instead of link with mbed-stubs
which is a group of stubs libraries.

- Previously all cellular header made as a part of the
mbed-headers-connectivity library. To make it easier to separate all
the cellular headers into the separate mbed-headers-cellular library
and keep them as part of cellular stub CMake. This makes the cellular
stubs more self-contained and improves the composition of the library.
- Update all unit test that depends on cellular headers with mbed-headers-cellular library

Move the connectivity cellular into the connectivity/cellular component
directory. So we can avoid duplicating the mbed-os source tree in a
central UNITTESTS folder.

- Add CMake configuration file into framework AT, device, common
unittest