[Failure Store] Test API keys and skip_unavailable with RCS1 (elastic#125782)

Adjust existing RCS1 tests to randomize using API keys for authorization
and `skip_unavailable` setting.

Followup on elastic#125252

Author: slobodanadamovic

x-pack/plugin/security/qa/multi-cluster/src/javaRestTest/java/org/elasticsearch/xpack/remotecluster/AbstractRemoteClusterSecurityFailureStoreRestIT.java

@@ -137,14 +137,14 @@ protected void setupTestDataStreamOnFulfillingCluster() throws IOException {
         }
     }
 
-    protected Response performRequestWithRemoteSearchUser(final Request request) throws IOException {
+    protected static Response performRequestWithRemoteSearchUser(final Request request) throws IOException {
         request.setOptions(
             RequestOptions.DEFAULT.toBuilder().addHeader("Authorization", headerFromRandomAuthMethod(REMOTE_SEARCH_USER, PASS))
         );
         return client().performRequest(request);
     }
 
-    protected Response performRequestWithUser(final String user, final Request request) throws IOException {
+    protected static Response performRequestWithUser(final String user, final Request request) throws IOException {
         request.setOptions(RequestOptions.DEFAULT.toBuilder().addHeader("Authorization", headerFromRandomAuthMethod(user, PASS)));
         return client().performRequest(request);
     }