[hf] read signal configs from env vars
- read all signal configs from env vars, but provide default values if none provided
- this allows us to give baseling configs that can be overriden (eg) to point signal at our own local fake signal server in load and/or e2e tests via env var injection
- in a future (better) design, we could accomplish something similar by providing different yaml config files: #410
- for now this should be enough to get load testin'!