[EKS] [request]: Allow pods with the same set of SGs attached to use one branch network interface

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**Tell us about your request**
We use security groups for pods. Currently each pod with security groups attached gets its own branch network interface. Since the number of interfaces per instance is limited, we are not able to launch as many pods as it is possible without using security groups.

For example, `c5.12xlarge` instance can have only `54` branch network interfaces. Normally, the `c5.12xlarge` instance type can have `234` pods scheduled on it, assuming they aren't associated with a security group.

The idea is to use one branch network interface for a few pods with the same set of security groups.

**Which service(s) is this request for?**
EKS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**
This will allow to utilize nodes more effectively and save our money.

**Are you currently working around this issue?**
Launching more nodes.

No repository files, tests, or entry points are named. Start by clarifying the EKS branch-network-interface behavior and acceptance criteria for sharing interfaces among pods with identical security groups; done means validating that pod capacity is no longer limited to one branch interface per pod without breaking security-group isolation.

Written by the indexing model from the issue text.