Add support for loadBalancerCluster to multi-cluster gateway service

[atoy3731]

The multicluster helm chart currently does not support the loadBalancerClass field. This is required by some loadbalance services like Tailscale.

Add a conditional loadBalancerClass key to the helm chart values and gateway svc.

Once merged, deploy the multicluster helm chart and configure .Values.gateway.loadBalancerClass to a value, ensure that is part of the resulting resource. Unset the value, ensure it is not part of the resulting resource.

Fixes #12100

[atoy3731]

I agree to the DCO for all the commits in this PR.

[atoy3731]

@adleong added the comment per those docs, but let me know if there's anything else I need to do on my side. Thank you!

[mateiidavid]

LGTM, thanks @atoy3731