Retrieve the externalId from the config

My previous change to enable the external id, had missed the part
that actually reads the externalId from the config. This change
is the final piece, all going well.

Signed-off-by: Brian Fletcher <brian@roadie.io>

.changeset/rich-mayflies-do.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-kubernetes-backend': patch
+---
+
+Fixes bug reading ExternalId from k8s backend config

plugins/kubernetes-backend/src/cluster-locator/ConfigClusterLocator.ts

@@ -44,7 +44,9 @@ export class ConfigClusterLocator implements KubernetesClustersSupplier {
           }
           case 'aws': {
             const assumeRole = c.getOptionalString('assumeRole');
-            return { assumeRole, ...clusterDetails };
+            const externalId = c.getOptionalString('externalId');
+
+            return { assumeRole, externalId, ...clusterDetails };
           }
           case 'serviceAccount': {
             return clusterDetails;