canbench as a blackbox performance monitoring tool

[ielashi]

In separate discussions with @frederikrothenberger and @jasonz-dfinity, there was the question of whether canbench supports benchmarking asynchronous methods. In these discussions, the idea we gravitated to was to create some kind of library support within canbench, where a developer can annotate a canister's endpoints, and then receive useful metrics/stats on the performance of these endpoints. These stats can be used in production for general performance monitoring, or in integration tests with PocketIC to benchmark async workflows.

cc @mbjorkqvist who had a similar request.