sztp: add signed-data-preferred to input

[glimchb]

example

diff --git a/examples/sztp/config/input.json b/examples/sztp/config/input.json
index 1bdc36c..0236172 100644
--- a/examples/sztp/config/input.json
+++ b/examples/sztp/config/input.json
@@ -3,6 +3,7 @@
       "hw-model" : "model-x",
       "os-name" : "vendor-os",
       "os-version" : "17.3R2.1",
+      "signed-data-preferred": [null],
       "nonce" : "BASE64VALUE="
     }
   }

from https://github.com/YangModels/yang/blob/main/experimental/ietf-extracted-YANG-modules/[email protected]#L79-L90

expect new fields in the response regarding owner in addition to conveyed-information, i.e.:

   <output
     xmlns="urn:ietf:params:xml:ns:yang:ietf-sztp-bootstrap-server">
     <conveyed-information>base64encodedvalue==</conveyed-information>
     <owner-certificate>base64encodedvalue==</owner-certificate>
     <ownership-voucher>base64encodedvalue==</ownership-voucher>
   </output>

[glimchb]

in go:

type AutoGenerated struct {
	IetfSztpBootstrapServerInput struct {
		HwModel             string        `json:"hw-model"`
		OsName              string        `json:"os-name"`
		OsVersion           string        `json:"os-version"`
		SignedDataPreferred []interface{} `json:"signed-data-preferred"`
		Nonce               string        `json:"nonce"`
	} `json:"ietf-sztp-bootstrap-server:input"`
}