AIP-72: Allow pushing and pulling XCom from Task Context

[kaxil]

Part of https://github.com/apache/airflow/issues/44481

Example DAG used:

"""
### DAG Tutorial Documentation
This DAG is demonstrating an Extract -> Transform -> Load pipeline
"""

from __future__ import annotations

import json
import textwrap

import pendulum

from airflow.models.dag import DAG
from airflow.providers.standard.operators.python import PythonOperator

with DAG(
    "sdk_tutorial_dag",
    default_args={"retries": 2},
    description="DAG tutorial",
    schedule=None,
    start_date=pendulum.datetime(2021, 1, 1, tz="UTC"),
    catchup=False,
    tags=["example"],
) as dag:
    dag.doc_md = __doc__

    def extract(**kwargs):
        ti = kwargs["ti"]
        data_string = '{"1001": 301.27, "1002": 433.21, "1003": 502.22}'
        ti.xcom_push("order_data", data_string)

    def transform(**kwargs):
        ti = kwargs["ti"]
        order_data = ti.xcom_pull(task_ids="extract", key="order_data")

        total_order_value = 0
        for value in order_data.values():
            total_order_value += value

        total_value = {"total_order_value": total_order_value}
        total_value_json_string = json.dumps(total_value)
        ti.xcom_push("total_order_value", total_value_json_string)

    def load(**kwargs):
        ti = kwargs["ti"]
        total_order_value = ti.xcom_pull(task_ids="transform", key="total_order_value")

        print(total_order_value)

    extract_task = PythonOperator(
        task_id="extract",
        python_callable=extract,
    )
    extract_task.doc_md = textwrap.dedent(
        """\
    #### Extract task
    A simple Extract task to get data ready for the rest of the data pipeline.
    In this case, getting data is simulated by reading from a hardcoded JSON string.
    This data is then put into xcom, so that it can be processed by the next task.
    """
    )

    transform_task = PythonOperator(
        task_id="transform",
        python_callable=transform,
    )
    transform_task.doc_md = textwrap.dedent(
        """\
    #### Transform task
    A simple Transform task which takes in the collection of order data from xcom
    and computes the total order value.
    This computed value is then put into xcom, so that it can be processed by the next task.
    """
    )

    load_task = PythonOperator(
        task_id="load",
        python_callable=load,
    )
    load_task.doc_md = textwrap.dedent(
        """\
    #### Load task
    A simple Load task which takes in the result of the Transform task, by reading it
    from xcom and instead of saving it to end user review, just prints it out.
    """
    )

    extract_task >> transform_task >> load_task

---

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