Improve SendWithDurabilityAsync testability using mocks

SendWithDurabilityAsync internally uses SendWithRetry to dispatch the original request before polling with observe operations to satisfy durability requirements.

However, the current implementation is not testable because a TaskCompletionSource cannot be passed into SendWithDurabillity, and in turn onto SendWithRetry, so it's not possible to mock the result. Ideally a TCS would be an optional parameter in the same way SendWithRetry exposes for testing.