backend: add leader epoch to partition api response

[bojand]

❯ http http://localhost:9090/api/topics/owlshop-orders-protobuf-plain/partitions
HTTP/1.1 200 OK
App-Build-Timestamp: 1729709471
Content-Type: application/json
Vary: Origin
Date: Wed, 23 Oct 2024 18:52:23 GMT
Content-Length: 255

{
    "topicName": "owlshop-orders-protobuf-plain",
    "partitions": [
        {
            "id": 0,
            "replicas": [
                0
            ],
            "offlineReplicas": null,
            "inSyncReplicas": [
                0
            ],
            "leader": 0,
            "waterMarkLow": 0,
            "waterMarkHigh": 7,
            "leaderEpoch": 1,
            "partitionLogDirs": [
                {
                    "brokerId": 0,
                    "partitionId": 0,
                    "size": 12929
                }
            ]
        }
    ]
}