tools: write offline_log_viewer kvstore/controller output to stdout

On stderr it gets mixed with logs.  On stdout it is
straightforward to capture and decode json for use in tests.

tools/offline_log_viewer/viewer.py

@@ -17,21 +17,32 @@
 
 
 def print_kv_store(store):
+    # Map of partition ID to list of kvstore items
+    result = {}
+
     for ntp in store.ntps:
         if ntp.nspace == "redpanda" and ntp.topic == "kvstore":
             logger.info(f"inspecting {ntp}")
             kv = KvStore(ntp)
             kv.decode()
             items = kv.items()
-            logger.info(json.dumps(items, indent=2))
+
+            result[ntp.partition] = items
+
+    # Send JSON output to stdout in case caller wants to parse it, other
+    # CLI output goes to stderr via logger
+    print(json.dumps(result, indent=2))
 
 
 def print_controller(store, bin_dump: bool):
     for ntp in store.ntps:
         if ntp.nspace == "redpanda" and ntp.topic == "controller":
             ctrl = ControllerLog(ntp)
             ctrl.decode(bin_dump)
-            logger.info(json.dumps(ctrl.records, indent=2))
+
+            # Send JSON output to stdout in case caller wants to parse it, other
+            # CLI output goes to stderr via logger
+            print(json.dumps(ctrl.records, indent=2))
 
 
 def print_kafka(store, topic, headers_only):