`Subscriber` must accept a `SubscriptionName` argument

[jabubake]

Currently, Subscriber requires a Subscription argument, which means retrieving the subscription using SubscriptionAdminClient after creating a SubscriptionName instance.
This is too much boiler plate code for the user to have to setup a Subscriber instance, we should accept a SubscriptionName argument.

@pongad

[jabubake]

My bad, was led astray by existing snippet.