Option to ignore server hosts for istio gateway source

[shysank]

What would you like to be added:

An optional annotation to some ignore hosts for istio gateway source. This annotation will be defined in istio's gateway object and hosts defined in this annotation (comma separated to ignore multiple hosts) will not be used to create endpoints

Why is this needed:

Sometimes there are server hosts in istio gateway whose dns is managed externally (terraform). External dns tries to reconcile these hosts resulting in error when trying to create the record since it would already have been created. Also, the record will be of a different type (geolocation record in route53) which means there is a potential for external dns to override this value.