Adding the support for key for trigger & output binding

[shrohilla]

• Adding the support for key for trigger & output binding as per customer demand :-

1. Key is always None in Python binding #326
2. The KafkaTrigger input parameter doesn't expose the (consumeResult.Message.)Key property  azure-functions-dotnet-worker#848

Kafka supported any library provide customer the support of reading key & value(message), so this is a required support for the customer

• Unit testing -- done
• E2E testing -- the language worker is still in-progress, will add the test case when that will be done

[shrohilla]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[TsuyoshiUshio]

You might add a simple unit testing for the new logic.

[krishna-kariya]

Should we set the Key as well in the constructor at line 110 in KafkaEventData.cs?

[shrohilla]

Should we set the Key as well in the constructor at line 110 in KafkaEventData.cs?

Yes, even if the key is null it will write the key as null and it's customer choice in his code if it wants to read that or not.

[lpapudippu]

LGTM