Support desc element in field

From [XEP-0004: Data Forms](https://xmpp.org/extensions/xep-0004.html#protocol-field):

> ...
> The <field/> element MAY contain any of the following child elements:
>
> <desc/>
> The XML character data of this element provides a natural-language
> description of the field, intended for presentation in a
> user-agent (e.g., as a "tool-tip", help button, or explanatory text
> provided near the field). The <desc/> element SHOULD NOT contain
> newlines (the \n and \r characters), since layout is the
> responsibility of a user agent, and any handling of
> newlines (e.g., presentation in a user interface) is unspecified
> herein. (Note: To provide a description of a field, it
> is RECOMMENDED to use a <desc/> element rather than
> a separate <field/> element of type "fixed".)
> ...

parsers/src/pubsub/owner.rs

@@ -233,6 +233,7 @@ mod tests {
                     type_: FieldType::ListSingle,
                     label: None,
                     required: false,
+                    desc: None,
                     options: vec![],
                     values: vec![String::from("whitelist")],
                     media: vec![],
@@ -280,6 +281,7 @@ mod tests {
                     type_: FieldType::ListSingle,
                     label: None,
                     required: false,
+                    desc: None,
                     options: vec![],
                     values: vec![String::from("whitelist")],
                     media: vec![],