Improve targetgroup.elbv2 target discovery

[michaelvl]

Is your feature request related to a problem?

When creating ALBs and thus TargetGroup resources I want to let the TargetGroup refer to Kubernetes Pods. Since Kubernetes Pods typically are dynamically created, I do not want to manually specify Pod IPs in the TargetGroup. Instead I would prefer something similar to how the aws-load-balancer controller does it using the TargetGroupBinding feature, which allow updating an AWS targetgroup based on Kubernetes Endpoints/Service resources.

See e.g. https://kubernetes-sigs.github.io/aws-load-balancer-controller/v2.1/guide/targetgroupbinding/targetgroupbinding/

A side note: Using the elbv2 controllers TargetGroup with an empty targets list and the aws-load-balancer controllers TargetGroupBinding does not work as the former removes targets added by the latter.

I see two architectural options here:

1. Keep the elbv2 controller as-is with 'plain' target definitions and use another controller for creating/updating the TargetGroup when Kubernetes Endpoints change.
2. Add a target lookup to the TargetGroup definition, i.e. conceptually include the aws-load-balancer controller feature in the e;bv2 controller.

The former keeps the elbv2 controller simpler but adds the need for another controller. The latter makes the elbv2 controller more 'self-contained' as it includes the target resolution but it means that the elbv2 controller becomes dependent on a non-AWS specific Endpoints resource.