net: sockets: tls: Implement TLS_DTLS_CID option

Add TLS_DTLS_CID socket option, which enables to use the Connection ID
extension for the DTLS session.

The option provides control of the use of CID with the `setsockopt()`
function. The value provided can disable, enable, and control whether to
provide a CID to the peer. It uses a random self CID (if told to provide
one to the peer) unless TLS_DTLS_CID_VALUE set previously.

Add TLS_DTLS_CID_VALUE to get or set the CID sent to the peer, if any.

Add TLS_DTLS_PEER_CID_VALUE to get the CID value provided by the peer,
if any.

Add TLS_DTLS_CID_STATUS to determine if CID used, and whether
bidirectional or one way.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>
Signed-off-by: Pete Skeggs <peter.skeggs@nordicsemi.no>