Summary: Added locust wrapper for load generation. The locust wrapper takes a locust config, which specifies every phase (users, duration) that should occur during the load test. The wrapper iterates through each phase, generating the locustfile, and running the locust command for the specified duration.

Test Plan: Started up a local version of hipstershop and ran `bazel run //demos/load_generation:load_generation -- --config_file ~/go/src/pixielabs.ai/pixielabs/demos/applications/hipster_shop/load_generation/load.pbtxt --host http://localhost:80`

Reviewers: philkuz, zasgar, kgandhi

Reviewed By: zasgar

Differential Revision: https://phab.corp.pixielabs.ai/D106

GitOrigin-RevId: f91e3adf652da3943ff928e0f4e77c53bc0bfd43