Define and document schema history topic messages schema

When Debezium detects and processes a schema change it emits a message into schema history topic. This message was never formally specified as an contract which means there is no evolution and compatibility rules in place.

It is is necessary to

• verify the current code the emits schema history messages if it conforms to schema naming, versioning etc.
• check each connector that the emitted messages conforms to the expectations
• document the message schema in documentation (ideally as a fragment that is shared by all connectors using the topic

That way we could expose the topic format as formal contract on which the users can rely.