Updating Amazon-AWS example DAGs to use XComArgs

[josh-fell]

Related to #10285

• Updated example DAG files such that xcom_pull() calls use an operator's .output property as well access of TaskInstance objects from context to use get_current_context() function
• Added comments to which task dependencies, if any, are handled and/or created via XComArgs for transparency
• Removed or refactored the default_args pattern where necessary as requested by @ashb (i.e. removed a separated default_args declaration for deference for declaration as part of the DAG object)

Note: There are several instances where the xcom_pull() call was not updated. These instances involve accessing a specific value within the XCom or calling user-defined macros with an XCom value. Reference #16618 for an open issue to enhance the XComArg functionality to provide similar behavior as the classic xcom_pull() method.

Note: Not all DAGs were tested functionally (i.e. with hard integrations to source systems and executed), however each DAG was tested to compile and generate a DAG graph as expected locally.

An detailed summary of all changes made as part of this PR can be found below:

DAG File	Converted xcom_pull()?	Other Updates?	Comments
airflow/providers/amazon/aws/example_dags/example_dms_full_load_task.py	Yes	Yes	Removed explicit task dependencies that are created via XComArgs. Refactored default_args pattern.
airflow/providers/amazon/aws/example_dags/example_dms_full_load_task.py	No	Yes	Refactored default_args pattern.
airflow/providers/amazon/aws/example_dags/example_emr_job_flow_automatic_steps.py	Yes	Yes	Removed explicit task dependencies that are created via XComArgs. Refactored default_args pattern.
airflow/providers/amazon/aws/example_dags/example_emr_job_flow_manual_steps.py	Yes **	Yes	** Current XComArg object does not support accessing specific values of an iterable XCom value elegantly. Did not update this occurrence: "{{ task_instance.xcom_pull(task_ids='add_steps', key='return_value')[0] }}". All other xcom_pull() calls have been updated. Removed explicit task dependencies that are created via XComArgs. Refactored default_args pattern.
airflow/providers/amazon/aws/example_dags/example_glacier_to_gcs.py	No	No	Current XComArg object does not support accessing specific values of an iterable XCom value. Could not update any occurrences in this DAG.
airflow/providers/amazon/aws/example_dags/example_google_api_to_s3_transfer_advanced.py	Yes	Yes	Removed explicit task dependencies that are created via XComArgs. Updated to use get_current_context().

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[github-actions]

The PR is likely OK to be merged with just subset of tests for default Python and Database versions without running the full matrix of tests, because it does not modify the core of Airflow. If the committers decide that the full tests matrix is needed, they will add the label 'full tests needed'. Then you should rebase to the latest main or amend the last commit of the PR, and push it with --force-with-lease.