Allow the user to set the key of the SourceRecord explicitly

[oscerd]

Currently, when the messages being polled, via this method:

SourceRecord record = new SourceRecord(sourcePartition, sourceOffset, topic, Schema.BYTES_SCHEMA, exchange.getMessage().getBody());

We don't set the Key of the record in the SourceRecord constructor which could be dangerous to send these data downstream non-keyed, therefore to tackle this, I'd suggest to all the user to set the Key explicitly, let's see a key of the exchange is available as a header, the user will just supply the name of the header in order to key the record with.
By default, I'd suggest to key the message by the messageId in case the user didn't set any configuration for the keys

author @omarsmak