net: sockets: Move socket vtable definition

Offloaded socket implementations need to create the socket operations
vtable, therefore need access to struct socket_op_vtable. So far this
has been defined in a private header, so implementations needed to add
the header location to the include path in CMake.

Therefore, move struct socket_op_vtable definition to the internal part
of the public socket header, so it's no longer needed to include a
private header. This is also more consistent with the rest of the public
header content, as for example macros needed to register a socket
implementation are already there.

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