Pass EndpointOpts during client creation

Actually, I think `EndpointLocator` will handle this defaulting for you.

*rummages through code*

As a matter of fact, `v3endpointLocator` does, but `v2endpointLocator` does not. Hrm.

Where do you think this belongs? It should be consistent across all `New{Service}` methods. Maybe we could provide an auxiliary method to apply defaults, that takes a Type argument, so it's done consistently everywhere?