Add optional resp attribute to abstract

Update teinteHeader.rng to allow an optional "resp" attribute on the <abstract> element (inside the define name="abstract"). This enables encoding responsibility/attribution metadata for abstracts in the TEI header schema.

Author: glorieux-f

teinteHeader.rng

@@ -1067,6 +1067,9 @@
     </define>
     <define name="abstract">
       <element name="abstract">
+        <optional>
+          <attribute name="resp"/>
+        </optional>
         <oneOrMore>
           <choice>
             <ref name="label.teiHeader"/>