rlqs: Support percentage/ratio-based rate limit strategies

RLQS RateLimitStrategy only supports absolute counts, not percentage/ratio-based limits

*Description*:
The Rate Limit Quota Service (RLQS) protocol expresses quota assignments via
`type.v3.RateLimitStrategy`, which currently supports only:

- `blanket_rule` (allow all / deny all)
- `requests_per_time_unit` (an absolute uint64 count per time unit)
- `token_bucket` (absolute max tokens + fill rate)

There is no way to express a quota as a percentage or ratio of some baseline
(e.g. "allow 10% of traffic to this bucket" or "shed 25% of requests"). This
would be useful for use cases like gradual rollout/ramp-up of a rate limiting
policy, proportional load shedding across buckets, or percentage-based traffic
shaping that scales automatically with incoming volume instead of requiring a
control plane to compute and continuously update absolute request-per-second
figures.

Requesting a new `RateLimitStrategy` variant (e.g. `PercentageRule` or
`ratio`) that lets the control plane express limits as a percentage/fraction
of observed request volume, similar to the existing percentage-based fault
injection / traffic shifting semantics elsewhere in Envoy (e.g.
`envoy.type.v3.FractionalPercent`).

[optional *Relevant Links*:]
> https://www.envoyproxy.io/docs/envoy/latest/api-v3/service/rate_limit_quota/v3/rlqs.proto
> https://www.envoyproxy.io/docs/envoy/latest/api-v3/type/v3/ratelimit_strategy.proto

Start by reading the RLQS protocol and strategy definitions in service/rate_limit_quota/v3/rlqs.proto and type/v3/ratelimit_strategy.proto, along with the referenced FractionalPercent semantics. Define the percentage or ratio-based RateLimitStrategy variant and its relationship to observed request volume; done means the protocol can express the requested percentage-based quota behavior.

Written by the indexing model from the issue text.