By default, advertises the HTTP server (if enabled).
Additional instances can be added by the user's application, example (for Apple HAP):
const struct mgos_dns_sd_txt_entry gizmo_txt[] = {
{.key = "c#", .value = MG_MK_STR("1")},
{.key = "ff", .value = MG_MK_STR("0")},
{.key = "pv", .value = MG_MK_STR("1.0")},
{.key = "id", .value = MG_MK_STR("11:22:33:44:55:66")},
{.key = "md", .value = MG_MK_STR("Fancy Gizmo 9000")},
{.key = "s#", .value = MG_MK_STR("1")},
{.key = "sf", .value = MG_MK_STR("1")},
{.key = "ci", .value = MG_MK_STR("8")}, // Switch
{.key = NULL},
};
mgos_dns_sd_add_service_instance("gizmo9000", "_hap._tcp", 8080, gizmo_txt);