Add API route that users can use for basic healthcheck processes (e.g. Load Balancer healthcheck)

• Add a new /_ping route on all APIs: public, admin, and metrics

• If healthy (HTTP server running) return HTTP 200 status and static body

• Should not require authentication to check the health of an otherwise accessible port (no information leakage)

• Should always be available to use (not behind a config flag)

• Should be logged at debug level only (silent at info)