Expose StreamingPullManager.load property via StreamingPullFuture

**Is your feature request related to a problem? Please describe.**
When implementing custom scaling mechanisms in worker-pool workloads, the only current working solution is to count active messages. This is cumbersome and also doesn't account for other reasons why a particular worker may be close to its limits. Using monitoring metrics is not always feasible and imposes longer aggregation periods.

**Describe the solution you'd like**
A public property on `StreamingPullFutire` that provides read-only getter to `__manager.load`

**Describe alternatives you've considered**
Counting total active messages and their cumulative size manually in code.