Feature Request: TargetGroupBinding for pods

Lattice has various deployment models.

Lattice is limited to 169.254.171.0/24, so only one ServiceNetwork can be associated per VPC.
Our deployment model seeks to connect “untrusted zones” between different domain and clusters.

Each cluster has a Lattice ServiceNetwork, and other clusters expose a Lattice Service and then share it in the form of RAM Share.
Currently, Gateway Controller has limited support for ServiceExport. Since CrossAccount is not being considered, IaC such as Terraform must be used.

Using two Kubernetes Controller and Terraform, causes a conflict in Lifecylce resource management and makes hard to manage resource.

Therefore, if the Gateway Controller manages only the TargetGroup like [TargetGroupBinding of the AWS LoadBalancer Controller](https://kubernetes-sigs.github.io/aws-load-balancer-controller/v2.8/guide/targetgroupbinding/targetgroupbinding/), and manages the remaining Service creation, RAM Share, Service Associate, and AuthPolicy through Terraform, I expect that the two life cycles can be clearly separated and managed.

I suggest creating a method to synchronize TargetGroup by explicitly entering the ARN of Lattice TargetGroup, such as TargetGroupBinding, rather than ServiceExport.

Start with the Gateway Controller's existing ServiceExport handling and compare it with the AWS Load Balancer Controller's TargetGroupBinding model. The proposed change should synchronize a TargetGroup from an explicitly supplied Lattice TargetGroup ARN, while Terraform retains management of Service creation, RAM Share, Service Associate, and AuthPolicy.

Written by the indexing model from the issue text.