aws-elasticloadbalancingv2-targets: InstanceTarget should register its instance as a connectable when added to a target group

### Describe the feature

When adding an ASG target, it calls registerConnectable on the targetGroup so that the ASG will accept connections from the load balancer. it seems like instance targets should do the same

### Use Case

If I make an instance a target of a load balancer, almost certainly I want to allow connections from the load balancer to said instance. :)

### Proposed Solution

See the difference between https://github.com/aws/aws-cdk/blob/81ef6650d123726ee01ec6cecba77d37244290e4/packages/%40aws-cdk/aws-autoscaling/lib/auto-scaling-group.ts#L1382-L1396 and https://github.com/aws/aws-cdk/blob/81ef6650d123726ee01ec6cecba77d37244290e4/packages/%40aws-cdk/aws-elasticloadbalancingv2-targets/lib/instance-target.ts#L26-L28

### Other Information

_No response_

### Acknowledgements

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

### CDK version used

2.27.0

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

Manjaro 21.3.0

Compare packages/@aws-cdk/aws-elasticloadbalancingv2-targets/lib/instance-target.ts with the registerConnectable handling in packages/@aws-cdk/aws-autoscaling/lib/auto-scaling-group.ts. Confirm how an InstanceTarget is added to a target group and ensure the instance is registered as a connectable; done means load balancer connections are allowed for instance targets without changing other target behavior.

Written by the indexing model from the issue text.