inject http client to rate limit collector and house keeping

[njasm]

closed #18

• Refactor Rate limit collector as described in Refactor RateLimit Collector and add a proper Description for prom crate #18
• Injecting HTTP client in collector's constructor like in GH Action runners and suggested here Add github actions runner collector #15 (comment)
• moved default_headers() function from collectors::github_runners to collectors module since that code is now used by both collectors.

[njasm]

r? @pietroalbini