Equivalent api for “set interface unnumbered [<intfc> use <intfc> | del <intfc>].” Details: https://docs.fd.io/vpp/17.04/clicmd_src_vnet.html#clicmd_set_interface_unnumbered

Supported by binary api:

@param client_index - opaque cookie to identify the sender
    @param context - sender context, to match reply w/ request
    @param sw_if_index - interface with an IP address
    @param unnumbered_sw_if_index - interface which will use the address
    @param is_add - if non-zero set the association, else unset it
*/
autoreply define sw_interface_set_unnumbered
{
  u32 client_index;
  u32 context;
  u32 sw_if_index;        /* use this intfc address */
  u32 unnumbered_sw_if_index;    /* on this interface */
  u8 is_add;
};

There is no api for read. Only cli (show interface cmd)