chore(load-testing): add k6 script for load testing

[halfwhole]

Problem

We'd like to test if our staging/production servers work well under load

Solution

Created a script for load testing that uses k6. The test lasts for roughly (slightly above) a minute, and the QPS is roughly (slightly under) NUM_VUS as configured in the script.

Notes:

• We can't put k6 in dev dependencies unfortunately because the underlying k6 module isn't written in JS, so the user will have to install k6 on their local machine
• k6 supports Datadog integrations, so we can do this in the future if we want to publish our stats from the load test

[gweiying]

lgtm!