rate_limit: reject unknown keys in the YAML configuration

The rate_limit YAML parser reads each setting by direct key lookup and ignores anything it does not recognize. A typo or a stale key name silently leaves the setting at its default, with no diagnostic at config load.

This is how `max-age` went unnoticed: every worked example in the documentation spelled the queue and ip-rep aging key with a hyphen, while the parser reads `max_age`, so those configurations parsed cleanly and did nothing.

Parsing sites:

* `plugins/experimental/rate_limit/sni_selector.cc` — `lists`, `ip-rep`, `selector` nodes
* `plugins/experimental/rate_limit/limiter.h` — `queue`, `metrics` sub-nodes
* `plugins/experimental/rate_limit/ip_reputation.cc` — `ip-rep` and `perma-block` sub-nodes

Each node should be validated against the set of keys it accepts, and an unrecognized key should fail the configuration load rather than be dropped.