**Description**

In similar fashion to https://github.com/apache/airflow/pull/4083, it would be helpful for Airflow to reattach itself to the ECS Task rather than letting another instance to start. However, instead of making this the default behavior, it would be better to use a `reattach` flag.

**Use case / motivation**

Allow Airflow the option to reattach to an existing ECS task when a restart happens, which would avoid having "rogue" tasks.
tests/providers/amazon/aws/operators/test_ecs.py
self.set_up_operator(launch_type=launch_type, tags=tags)  
        client_mock = self.aws_hook_mock.return_value.get_conn.return_value
        client_mock.run_task.return_value = RESPONSE_WITHOUT_FAILURES
        self.ecs.execute(None)
        self.aws_hook_mock.return_value.get_conn.assert_called_once()
        extend_args = {}
        if launch_type:
            extend_args['launchType'] = launch_type
        if launch_type == 'FARGATE':
            extend_args['platformVersion'] = 'LATEST'
        if tags:
            extend_args['tags'] = [{'key': k, 'value': v} for (k, v) in tags.items()]
        client_mock.run_task.assert_called_once_with(
            cluster='c',
            overrides={},
            startedBy=mock.ANY,  
            taskDefinition='t',
            group='group',
            placementConstraints=[{'expression': 'attribute:ecs.instance-type =~ t2.*', 'type': 'memberOf'}],
            placementStrategy=[{'field': 'memory', 'type': 'binpack'}],
            networkConfiguration={
                'awsvpcConfiguration': {'securityGroups': ['sg-123abc'], 'subnets': ['subnet-123456ab']}
            },
            propagateTags='TASK_DEFINITION',
            **extend_args,
        )
        wait_mock.assert_called_once_with()