Add Gitlab::Redis::RateLimiting Redis instance

This instance is not used yet, but will soon be used for rate limiting via Rack::Attack and Gitlab::ApplicationRateLimiter. Having this configuration option in place will allow us to build on that, and also create merge requests to support this option in Omnibus and our Helm charts.

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The second commit simplifies the process of adding a new Redis instance:

1. config/README.md now has a table for the Redis instances with a single list of instructions that apply to all instances.
2. There are now specs to ensure that each Redis instance has a default URL set when no Redis config at all is present.
3. Instances that use a fallback instance for config will automatically use the default URL of their fallback instance. They should not define their own default URL.
4. The .gitignore file does not need to be updated for each new Redis instance.
5. The example config files are now gone as request.yml.example should be sufficient.

For gitlab-com/gl-infra/scalability#1247.