XFA - Save filled data in the pdf when downloading the file (Bug 1716288)

- when binding (after parsing) we get a map between some template nodes and some data nodes;
  - so set user data in input handlers in using data node uids in the annotation storage;
  - to save the form, just put the value we have in the storage in the correct data nodes, serialize the xml as a string and then write the string at the end of the pdf using src/core/writer.js;
  - fix few bugs around data bindings:
    - the "Off" issue in Bug 1716980.

src/core/document.js

@@ -963,6 +963,13 @@ class PDFDocument {
     this.xfaFactory.setFonts(pdfFonts);
   }
 
+  async serializeXfaData(annotationStorage) {
+    if (this.xfaFactory) {
+      return this.xfaFactory.serializeData(annotationStorage);
+    }
+    return null;
+  }
+
   get formInfo() {
     const formInfo = {
       hasFields: false,