Make Serialization & De-serialization stricter with versioning with Task SDK

[kaxil]

Currently the serialization and de-serialization logic lives in airflow/serialization in the Core. With Airflow 3 and the separation of Task SDK, we will need to make serialization and its versioning much stricter.

We should bump the current DAG serialization version to 2.

The serialization code should live closer to language-specific Task SDK as it knows best how to serialize objects in a language to a JSON-formatted string.

The Core/scheduler will contain the de-serialization code -- and it does need to be language specific as it contains only the info needed by the scheduler.

The contract between those two is the schema.json file that contains the serialization. Both the client and server could support multiple versions at a time.