Adds services.hyperhive.otel.collector.{enable,port}. No implementation
yet - this commit only declares the surface, so the option docs and the
default-off invariant can be reviewed on their own.
The reason the collector exists is the credential: without it every agent
needs headersCredential to reach the upstream, and the harness delivers
that token into the agent's own settings.json where the agent can read
it. The option text states the cost too - the collector becomes a
dependency in an export path that is currently direct.
