aws_elasticloadbalancingv2: Can't interact with the Network or Application Target Group from EcsTarget

### Describe the feature

Maybe I've missed something, but if you do something like the following:

```python3

ecs_target = ecs.EcsTarget(
container_name="container_name",
container_port=80,
new_target_group_id="TargetGroup80",
listener=ecs.ListenerConfig.network_listener(
listener=nlb_listener_80,
port=80,
deregistration_delay=Duration.seconds(10),
),
)

# Register the target with the target group
nlb_flask_fargate_service.service.register_load_balancer_targets(ecs_target)
```

There's no way to further interact with the `lb.NetworkTargetGroup` or `lb.ApplicationTargetGroup`.

If you try to import the `TargetGroup` via the following, you need the full ARN which the above `EcsTarget` construct doesn't give you (just the `new_group_id`)

```python3

def import_80_target_group(self) -> None:
target_group_80 = lb.NetworkTargetGroup.from_target_group_attributes(
self,
"TargetGroup80Imported",
target_group_arn=ecs_target.new_target_group_id,
load_balancer_arns=nlb_flask_fargate_service.load_balancer.load_balancer_arn,
)
```

### Use Case

Add custom metrics / monitoring / alarms / scaling to the target group:

```python3

target_group_80.metrics.custom(metric_name="RequestCount", period=Duration.minutes(1)).create_alarm(
self,
"HighNumRequestsAlarm",
evaluation_periods=1,
threshold=20,
comparison_operator=cw.ComparisonOperator.GREATER_THAN_OR_EQUAL_TO_THRESHOLD,
alarm_description="ECS Scale up on Number of Incoming Requests if number of requests is greater than 20 in a minute",
)
```

```python3

scaling.scale_on_metric(
"TargetResponseTimeScaling",
metric=self.flask_nlb_fargate_service.target_group.metrics.target_response_time(),
scaling_steps=[
appscaling.ScalingInterval(lower=0, change=-1), # Decreases tasks if requests are low.
appscaling.ScalingInterval(lower=10.0, change=+1), # Scale out if response time > 10s, add a task
appscaling.ScalingInterval(lower=20.0, change=+2), # Scale out more if > 20s, add 2 tasks
],
adjustment_type=appscaling.AdjustmentType.CHANGE_IN_CAPACITY,
)
```

### Proposed Solution

Expose the underlying `TargetGroup` construct via a property on `EcsTarget` or at least the full ARN.

### Other Information

_No response_

### Acknowledgements

- [ ] I may be able to implement this feature request
- [ ] This feature might incur a breaking change

### CDK version used

2.180.0

### Environment details (OS name and version, etc.)

MacOS

Start at the EcsTarget construct and register_load_balancer_targets entry points, then inspect how the NetworkTargetGroup and ApplicationTargetGroup constructs are created. Compare whether exposing the underlying TargetGroup or its full ARN best supports the metrics, alarms, and scaling examples; done means callers can interact with that target group without reconstructing its ARN.

Written by the indexing model from the issue text.