Resolve "Rate limit the GET /api/v4/namespaces/:id/exists API"

What does this MR do and why?

For #366822 (closed)

This MR introduces rate limiting for the GET /api/v4/namespaces/:id/exists API at the rate of maximum 20 requests per 1 minute.

This API is used to recommend a unique url for the group when creating a new group.

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