schema: Support References without type

There are some examples in the OSV database which have untyped
references. Currently these fail to parse. For example:
https://www.googleapis.com/download/storage/v1/b/osv-vulnerabilities/o/Debian%2FCVE-2018-11212.json?generation=1758701673567991&alt=media

Author: Jarrah-TLR

src/schema.rs

@@ -442,7 +442,7 @@ pub enum ReferenceType {
 pub struct Reference {
     /// The type of reference this URL points to.
     #[serde(rename = "type")]
-    pub reference_type: ReferenceType,
+    pub reference_type: Option<ReferenceType>,
 
     /// The url where more information can be obtained about
     /// the vulnerability or associated the fix.