KafkaEventData support for isolated extension

### Description

Per https://github.com/Azure/azure-functions-dotnet-worker/issues/1720 and https://github.com/Azure/azure-functions-dotnet-worker/issues/1399, there is a need for a wrapper POCO (`KafkaEventData`) that is currently only available in the WebJobs extension. We should assess bringing an equivalent to isolated.

Because the type is extension-defined, it does not live in a shared referenceable location. We need to figure out how we want to navigate that. The behavior described in the two linked issues also needs assessment to ensure the representation being passed through the gRPC layer is appropriate. Of note, it seems that things are represented by a [`KafkaTriggerInput`](https://github.com/Azure/azure-functions-kafka-extension/blob/dev/src/Microsoft.Azure.WebJobs.Extensions.Kafka/Trigger/KafkaTriggerInput.cs) which gets converted into either `KafkaEventData` or `KafkaEventData[]`.

Work here may be required of the Kafka WebJobs extension.

Start with linked issues 1720 and 1399, then read the KafkaTriggerInput.cs entry point in the Kafka WebJobs extension. Trace how it becomes KafkaEventData or KafkaEventData[] and how that representation crosses gRPC. Done means the isolated wrapper, extension ownership, and required representation changes are clearly agreed.

Written by the indexing model from the issue text.