dual stack ingress: support IPAM pools

[guydc]

How to categorize this issue?
/area networking
/kind enhancement
/platform aws

What would you like to be added:
Support static IPv6 addresses for dualstack LBs. To achieve this, gardener may support allocation of IPv6 CIDRs to the VPC and public subnet from a user-managed IPAM pool.

Why is this needed:
When a user-managed IPAM pool is used, the LB IPv6 addresses are allocated from a stable range that is decoupled from the cluster's lifecycle (not released in case of VPC deletion). Users may also select specific addresses from within these ranges, using the following annotation: service.beta.kubernetes.io/aws-load-balancer-ipv6-addresses. These ranges/addresses can be communicated to consumers for allowlisting purposes.