KAFKA-4108; Improve DumpLogSegments offsets-decoder output format

This PR improves the output format of DumpLogSegments when the `--offset-decoder` option is used for consuming `__consumer_offsets`, especially when it comes to group metadata.

An example of the partial output with existing formatting:

```
key: metadata::console-consumer-40190 payload: consumer:range:1:{consumer-1-20240b92-fbf4-44d5-bf8c-66b6d70c9948=[foo-0]}
```

An example of the same output with suggested formatting:

```
key: {"metadata":"console-consumer-40190"} payload: {"protocolType":"consumer","protocol":"range","generationId":1,"assignment":"{consumer-1-20240b92-fbf4-44d5-bf8c-66b6d70c9948=[foo-0]}"}
```

Author: Vahid Hashemian <[email protected]>

Reviewers: Jason Gustafson <[email protected]>

Closes apache#1937 from vahidhashemian/KAFKA-4108

core/src/main/scala/kafka/tools/DumpLogSegments.scala

@@ -334,8 +334,12 @@ object DumpLogSegments {
         }
       }.mkString("{", ",", "}")
 
-      val keyString = s"metadata::$groupId"
-      val valueString = s"$protocolType:${group.protocol}:${group.generationId}:$assignment"
+      val keyString = Json.encode(Map("metadata" -> groupId))
+      val valueString = Json.encode(Map(
+          "protocolType" -> protocolType,
+          "protocol" -> group.protocol,
+          "generationId" -> group.generationId,
+          "assignment" -> assignment))
 
       (Some(keyString), Some(valueString))
     }