Add custom or 'prepared' diff field to synchronize module

This PR depends on pull requeest ansible/ansible#14105

rsync has a custom diff output that cannot easily be expressed as
`/usr/bin/diff before after`

files/synchronize.py

@@ -389,6 +389,12 @@ def main():
         out_lines=out_clean.split('\n')
         while '' in out_lines: 
             out_lines.remove('')
+        if module._diff:
+            diff = {'prepared': out_clean}
+            return module.exit_json(changed=changed, msg=out_clean,
+                                    rc=rc, cmd=cmdstr, stdout_lines=out_lines,
+                                    diff=diff)
+        else:
             return module.exit_json(changed=changed, msg=out_clean,
                                     rc=rc, cmd=cmdstr, stdout_lines=out_lines)