[Thread][API] add API for ephemeral key.

This CL adds the API for starting/stopping the ephemeral key mode, and
subscribing to the ephemeral key state changes.

Ephemeral key mode is started with a limit amount of time to allow
Thread credential sharing.

Bug: 348323500
Test: atest CtsThreadNetworkTestCases
Change-Id: I540c88ec4668291358fbe87a678d8f9d31a67e54
7 files changed