Move XCom serialization and deserialization methods to the Task SDK

[kaxil]

Currently, as part of AIP-72, we implemented XCom API endpoints in the Task Execution API Server that expect a JSON-serialized string

airflow/airflow/api_fastapi/execution_api/routes/xcoms.py

Lines 114 to 138 in 9be5971

	def set_xcom(
	dag_id: str,
	run_id: str,
	task_id: str,
	key: str,
	value: Annotated[
	str,
	Body(
	description="A JSON-formatted string representing the value to set for the XCom.",
	openapi_examples={
	"simple_value": {
	"summary": "Simple value",
	"value": '"value1"',
	},
	"dict_value": {
	"summary": "Dictionary value",
	"value": '{"key2": "value2"}',
	},
	"list_value": {
	"summary": "List value",
	"value": '["value1"]',
	},
	},
	),
	],

This was done so that we can have the same interface for multi-language support (Python, Go, Java etc) for Task SDK. The contract is simple: API Server always deals with JSON-formatted string --- and the responsibility of serialization and de-serialization of Native objects to string and back lies to the language-specific clients.

In order to do that we should move the current Serialization and de-serialization logic to clients. This will also allow storing and using XCom backends correctly, which can also handle different languages.

As part of this task, we should also figure out how we can avoid serialization and deserialization from SQLAlchemy as we use JSON type for value column --- which leads to double / triple serialization

1. Serialization from Task SDK and sent to API Server
2. API Server calls XCom.serialize
3. SQLAlchemy JSON type serializes it further

Result of Triple serialization: