Add a way to check rate limits without using quota

The grpc [RateLimitService](https://github.com/envoyproxy/envoy/blob/main/api/envoy/service/ratelimit/v3/rls.proto) has only one method `ShouldRateLimit` and `hits_addend` is assumed to be 1 when no value has been explicitly set.

**Why would a person want to get quota without incrementing it?**

One example is if you're dealing with expensive requests but you don't know how expensive they are until _after_ they've run. Maybe you add up the CPU seconds spent serving a request and increment it at the end of the request. You'd still need to check (but not increment) at the beginning of requests whether the CPU-second quota has been exhausted by previous requests during the current time unit.

A hacky workaround today might be to make sure that expensive requests are measured in large numbers (like hundreds of thousands) and check quota by incrementing a very small number (like 1) that might effectively act as if you haven't incremented it at all.

**How might this be implemented?**

Seems like there are a few possible approaches to getting quota without changing it:
- Do something clever and subtle like saying that a `hits_addend` equal to uint32_max means zero
- Add some kind of a `check_only` flag to the `RateLimitRequest` message
- Make a separate `IsRateLimited` method on `RateLimitService`
- Make a separate non-standard grpc service specific to this rate limiter implementation so the `RateLimitService` in envoy proxy doesn't need to change