Missing ServiceLoader manifest (for Kafka >= 3.6)

Kafka Connect has added new plugin discovery with ServiceLoader since 3.6.
If the plugin doesn't register itself using ServiceLoader KafkaConnect reports:

```
One or more plugins are missing ServiceLoader manifests may not be usable with plugin.discovery=service_load: [
file:/opt/plugins/pubsub-group-kafka-connector-1.2.0.jar com.google.pubsub.kafka.sink.CloudPubSubSinkConnector sink 3.7.0
file:/opt/plugins/pubsub-group-kafka-connector-1.2.0.jar com.google.pubsub.kafka.source.CloudPubSubSourceConnector source 3.7.0
file:/opt/plugins/pubsub-group-kafka-connector-1.2.0.jar com.google.pubsublite.kafka.sink.PubSubLiteSinkConnector sink 3.7.0
file:/opt/plugins/pubsub-group-kafka-connector-1.2.0.jar com.google.pubsublite.kafka.source.PubSubLiteSourceConnector source 3.7.0
Read the documentation at https://kafka.apache.org/documentation.html#connect_plugindiscovery for instructions on migrating your plugins to take advantage of the performance improvements of service_load mode. To silence this warning, set plugin.discovery=only_scan in the worker config.
```

I would like to set [plugin.discovery](https://kafka.apache.org/documentation.html#connectconfigs_plugin.discovery) to service_load to speed up the cluster startup. But it requires proper plugin registration with ServiceLoader.

Now it's just a WARN but it could change in the future version of Kafka.