Handle Custom XCom Backend on Task SDK

[kaxil]

Follow-up of #45231 to move XCom Backend on Task SDK.

As mentioned on #45231, it will be the client's responsibility to serialize and deserialize before sending it to the API server to support multiple language.

For XCom Backends, where you want to stop sending MBs or GBs of data to the DB (via the API-server), we will handle this on the client side so the data is sent from the Worker (and client) itself.

https://airflow.apache.org/docs/apache-airflow/stable/core-concepts/xcoms.html#custom-xcom-backends