DISCUSS(kafka-logger): the key field should be optional

[Yuelin Zheng]

Hi, Community,


In the schema of the kafka-logger plugin, the key field is mandatory, as shown 
below:


```
......
    properties = {
        ......
        key = {type = "string"},
        ......
    },
    required = {"broker_list", "kafka_topic", "key"}
......
```


I think the key field should be optional, because when a message is sent to the 
Kafka service, the key can be specified or empty. So when the key is empty, it 
can be used normally.


The key should be optional in the schema：


```
......
    properties = {
        ......
        key = {type = "string"},
        ......
    },
    required = {"broker_list", "kafka_topic"}
......
```


Related issue：https://github.com/apache/apisix/issues/2778