MT#9671 Add search of subscribers by customer external_id too. Again.

This reverts commit c52040353f.

Tests failed also with this revert, so it seems as safe
to return this functionality.

lib/NGCP/Panel/Controller/API/Subscribers.pm

@@ -83,6 +83,19 @@ class_has 'query_params' => (
                 },
             },
         },
+        {
+            param => 'customer_external_id',
+            description => 'Filter for subscribers of a specific customer external_id.',
+            query => {
+                first => sub {
+                    my $q = shift;
+                    return { 'contract.external_id' => { like => $q } };
+                },
+                second => sub {
+                    return { join => 'contract' };
+                },
+            },
+        },
         {
             param => 'is_pbx_group',
             description => 'Filter for subscribers who are (not) pbx_groups.',