test: add a stress test for the https outcalls feature

[mihailjianu1]

This test adds an update message to the already existing proxy_canister, which performs count concurrent outcalls.

However, as the number of canister messages is limited to 500, the requests have to be split into batches.

All requests are lightweight both in therms of request size and response size.

The qps in these conditions (for 500, 1000 and 2000 "concurrent" requests) was around 150, with a latency of 2-3 seconds.