Discord, Slack, and Webhook providers now use a single HTTPProvider
struct with a PayloadFunc for the only part that differs. Centralizes
response body handling and adds HTTP status code checking (4xx/5xx
now return errors instead of being silently ignored).
Email and Ntfy keep separate implementations (different protocols).
Adding a new HTTP-based alert provider is now a one-line PayloadFunc.
lerko
d6f33a4d1f