Add LoadBalancer Ref support to VPCEndpointServiceConfiguration

[a-mcloughlin]

Issue #, if available: N/A

Description of changes:
Adding a GatewayLoadBalancerRef and NetworkLoadBalancerRef to the VpcEndpointServiceConfiguration crd,
referencing elbv2 LoadBalancer arn

Structure:

apiVersion: ec2.services.k8s.aws/v1alpha1
kind: VPCEndpointServiceConfiguration
metadata:
  name: test
spec:
  acceptanceRequired: false
  networkLoadBalancerRefs:
    - from:
        name: test

Tested locally, everything works as intended, and existing functionality is not impacted.

If a ref to a load balancer of the wrong type is passed to the ref fields (e.g. ALB), the aws api recognises the invalid LB type and logs the error cleanly

Thanks for taking the time to review!
I appreciate any feedback on this PR

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ack-prow]

Hi @a-mcloughlin. Thanks for your PR.

I'm waiting for a aws-controllers-k8s member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[ack-prow]

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