feat(slack) add rate limiting

[rkettelerij]

Description

Avoid messages like:

"error": "slack rate limit exceeded, retry after 1s"}
2024-06-24 18:25:39.149Z github.com/PDOK/uptime-operator/internal/service.(*Slack).Send
2024-06-24 18:25:39.149Z        /workspace/internal/service/slack.go:31
2024-06-24 18:25:39.149Z github.com/PDOK/uptime-operator/internal/service.(*UptimeCheckService).logMutation
2024-06-24 18:25:39.149Z        /workspace/internal/service/service.go:135
2024-06-24 18:25:39.149Z github.com/PDOK/uptime-operator/internal/service.(*UptimeCheckService).Mutate
2024-06-24 18:25:39.149Z        /workspace/internal/service/service.go:79
2024-06-24 18:25:39.149Z github.com/PDOK/uptime-operator/internal/controller.(*IngressRouteReconciler).Reconcile```

Type of change

• Improvement of existing feature

Checklist:

• I've double-checked the code in this PR myself
• I've left the code better than before (boy scout rule)
• The code is readable, comments are added that explain hard or non-obvious parts.
• I've expanded/improved the (unit) tests, when applicable
• I've run (unit) tests that prove my solution works
• There's no sensitive information like credentials in my PR