[Adapter] Create Adapters for Channels and Subscriptions

[jcscottiii]

In lib/gcpspanner/spanneradapters, create new adapters to expose high-level methods for managing channels (e.g., CreateChannel, GetChannelForUser) and subscriptions (e.g., CreateSubscription, ListSubscriptionsForUser). This layer will abstract the underlying spannerMapper logic. This PR must include unit tests that mock the gcpspanner client.