Add configurable Redis key prefixes

[jacobmstein]

Redis keys are conventionally namespaced using colons to avoid naming collisions and improve readability. Additionally, bare keys often make it difficult to view database contents from GUI clients, many of which group and filter keys by namespace.

For those reasons, this pull request introduces a RedisRateLimitOptions class with a KeyPrefix property.

[timonmasberg]

Actually, wouldn't it be enough to just remove the _ when we call the policy store, then we could use ClientPolicyPrefix for the counter key as well without introducing another option?

[jacobmstein]

Actually, wouldn't it be enough to just remove the _ when we call the policy store

I suppose, however I find it unreasonable to alter its current behavior as a feature. In that case, a delimiter option (default _) needs to be added, which I'd be happy to do.

[ShawnWang6]

Why not merge this MR? I'm waiting for this feature.

[baramustafa]

Will this ever get merged in? Would be very handy if it could.