Add feature "sort"

This feature indicates support for "locale", "sort-by", and "direction".

Fixes: #28

draft-ietf-netconf-list-pagination.xml

@@ -227,6 +227,8 @@
         <t>The sorting can furthermore be configured with a locale for
           sorting. This is done by setting the "locale" parameter
           (see <xref target="locale"/>).</t>
+        <t>The feature "sort" is used to indicate support for the query
+          parameters "locale", "sort-by", and "direction".</t>
 
         <section title='The "where" Query Parameter' anchor="where" toc="exclude">
           <dl newline="true">

ietf-list-pagination.yang

@@ -71,6 +71,13 @@ module ietf-list-pagination {
       "RFC XXXX: List Pagination for YANG-driven Protocols";
   }
 
+  // Features
+
+  feature sort {
+    description
+      'This feature indicates that the parameters "locale", "sort-by",
+       and "direction" are supported.';
+  }
 
   // Annotations
 
@@ -112,6 +119,7 @@ module ietf-list-pagination {
   }
 
   md:annotation locale {
+    if-feature "sort";
     type string;
     description
       "This annotation contains the locale used when sorting.
@@ -148,6 +156,7 @@ module ietf-list-pagination {
   }
 
   identity locale-unavailable {
+    if-feature "sort";
     base list-pagination-error;
     description
       "The 'locale' query parameter input is not a valid
@@ -187,12 +196,14 @@ module ietf-list-pagination {
            statement applied to it (see RFC XXXX).";
       }
       leaf locale {
+        if-feature "sort";
         type string;
         description
           "The 'locale' parameter indicates the locale which the
            entries in the working result-set should be collated.";
       }
       leaf sort-by {
+        if-feature "sort";
         type union {
           type string {
             // An RFC 7950 'descendant-schema-nodeid'.
@@ -219,6 +230,7 @@ module ietf-list-pagination {
            the end (e.g., after all nodes having values).";
       }
       leaf direction {
+        if-feature "sort";
         type enumeration {
           enum forwards {
             description