Account REST API: Add a new endpoint to delete external IDs

The REST endpoint is added as:
'POST /accounts/{account-id}/external.ids:delete'
The target external ids must be provided in the request body.

Change-Id: Ieef322d6bddc1f3659536081b3f3514474b7162a

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/accounts/AccountApi.java

@@ -81,6 +81,7 @@ public interface AccountApi {
   void index() throws RestApiException;
 
   List<AccountExternalIdInfo> getExternalIds() throws RestApiException;
+  void deleteExternalIds(List<String> externalIds) throws RestApiException;
 
   /**
    * A default implementation which allows source compatibility
@@ -233,5 +234,10 @@ public interface AccountApi {
     public List<AccountExternalIdInfo> getExternalIds() {
       throw new NotImplementedException();
     }
+
+    @Override
+    public void deleteExternalIds(List<String> externalIds) {
+      throw new NotImplementedException();
+    }
   }
 }