Add issuable searches to search rate limit

What does this MR do and why?

Rate limits issuable searches so that we can re-enable searching made by anonymous users. This uses the existing search rate limit settings.

This is behind a feature flag rate_limit_issuable_searches.

Note: I also considered rate limiting at the finder level, but I think there may be other places that use the finder but we don't want to include in the limit. For example, in the recent issues autocomplete finder, it adds a scope with a list of issue IDs so the searches here are fast and we do not need to limit them.

Related to #340836

How to set up and validate locally

1. Enable rate_limit_issuable_searches
2. (Optional) Lower the search rate limit in Admin > Settings > Network > Search rate limits
3. Load a page like http://127.0.0.1:3000/dashboard/issues?scope=all&state=opened&search=test many times to go over the limit
4. Get a 429 response

MR acceptance checklist

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