Make the SerializedDAG & SerializedBaseOp be independent of Task SDK's DAG & BaseOP

[kaxil]

Currently, models.BaseOperator and models.DAG use the Task SDK classes.

This was done to maintain backwards-compat and to get 3.0 in a timely manner.

However, we don't want to depend on Task SDK in the Scheduler or the API-server (airflow-core package) - at least not a required dependency.

We only need to ensure we have enough information for the Scheduler & API-server for a DAG & the Operator. This is done via the SerializedDAG & SerializedBaseOp.

So this task involves: figuring out how we can consolidate: SerializedDAG, models.DAG by not inheriting Task SDK; and same for SerializedBaseOp and models.BaseOperator

To do (not ordered by dependency):

• [x] Timetables
• [ ] Assets
• [ ] DAGNode
• [x] NOTSET and ArgNotSet
• [ ] DeadlineAlert (and related things?) - Currently WIP
• [x] BaseOperatorLink
• [x] ExpandInput
• [ ] Partitions
• [ ] Clean up remaining SDK references in Core
• [ ] Take airflow.serialization.serialized_objects apart
• [ ] Move MappedOperator to airflow.serialization.definitions

[uranusjr]

• [x] BaseOperator
• [x] MappedOperator
• [x] DAG

[kaxil]

• [x] Param
• [x] ParamsDict
• [x] TaskGroup

[uranusjr]

Not done

[amoghrajesh]

@uranusjr we should also add an issue for BaseTrigger/BaseEventTrigger?

[uranusjr]

We should eventually, but I’m not sure what to do with triggerer things yet. Let’s put them off for now until we figure triggerer and dag processor split out.