net: sockets: tls: Decouple TLS sockets from net_context

Separate TLS sockets implementation from net_context layer. Instead of
calling net_context functions directly, create an underlying TCP/UDP
socket, and call socket functions on it instead. This results in a
simpler design, where we don't need to duplicate specific parts of code
from the native socket implementation. Additionally, this allows to use
a different TCP/UDP stack underneath (i. e. when sockets offloading is
used).

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>