rules: rate limiting

[mrl5]

closes #18

overview

• implements sliding window algorithm (described in https://blog.cloudflare.com/counting-things-a-lot-of-different-things/)
• allocates two fixed size buffers (std::collections::HashMap). Size is configurable by capacity param
• buffers are controlled by single thread. tokio::sync::mpsc::channel is used for communication with request handlers aka "hot path"

pingoo.yml interface

rules:
  rate_limit_api_routes:
    expression: http_request.path.starts_with("/api/")
    actions:
+     - action: limit
+   limit:
+     max: 10
+     period: 60      # in seconds
+     capacity: 1024  # number of IPs that can be tracked