warn when LoadBalancer has multiple fallbacks

[dfong]

apigee LoadBalancer documentation says that a LoadBalancer should have at most one fallback. https://cloud.google.com/apigee/docs/api-platform/deploy/load-balancing-across-backend-servers#settingloadbalanceroptions-isfallback

i would like apigeelint to issue a warning if the rule is violated. for example, this should generate a warning:

<LoadBalancer>
    <Server name="gateway-east"><IsFallback>true</IsFallback></Server>
    <Server name="gateway-east"><IsFallback>true</IsFallback></Server>
</LoadBalancer>

i would prefer that apigeelint treat this as a warning rather than an error, since the rule is not currently enforced by the control plane.