Convey original operation type when using flattening SMTs

The problem

Currently there's no way to inform whether the message operation is, this information is lost along the way through the `UnwrapFromMongoDbEnvelope`, we can only distinguish between deletions or not.

The scenario

I want to be able to tell whether the message is an update or insert thus easing logic within the Sink Connector.
The rationale is about optimizing the Sink in a way it doesn't have to look for an item before knowing whether it should insert or update, this can potentially save a great number of queries in the Sink side.
Basically avoid `saveOrUpdate` situations by knowing exactly which operation to run.

Past proposed solution (rejected)

Currently the value schema has a metadata name which informs the server, db and collection, I want to add on it the operation as in `io.debezium.data.Envelope.Operation`
The current value looks like: `serverX.dbA.c1`
Proposed value would be:

• `serverX.dbA.c1.r` for initial read
• `serverX.dbA.c1.c` for creation
• `serverX.dbA.c1.u` for updates

This is very similar to the initialSync key in the non cdc way.

Proposed solution

Add a new header into the kafka record, finally being available in the message itself, this header will be optional via a transformer flag (MONGO_OPERATION_HEADER) and will respect the `io.debezium.data.Envelope.Operation` enum.
The proposed header name is `application/debezium-operation`

Implementation details

This can be an optional feature within the transformer thus not breaking BC