Add AWS client component

The new AWS client component added by this commit is providing some of
the libraries from the AWS IoT device SDK (Backoff_algorithm, CoreMQTT,
CoreJSON and CoreHTTP), as well as a transport interface to simpify
the use of these libraries.
We are not using the Cmake provided with the AWS IoT device SDK as it
would compile a lot of libraries which we don't use as they are
 Freertos specific.

Signed-off-by: Vincent Coubard <vincent.coubard@arm.com>
Signed-off-by: Hugo L'Hostis <hugo.lhostis@arm.com>