add params thresholds to rdv events health endpoint

Created by: adipasquale

https://trello.com/c/iQ07hFf3/1044-mettre-en-place-une-alerte-en-cas-de-peu-de-mails-envoy%C3%A9s-le-matin

permettra de mettre en place des alertes depuis updown.io sur l'url https://www.rdv-solidarites.fr/health_checks/rdv_events_stats.json?min_mails=15&min_sms=40

res visible sur https://demo-rdv-solidarites-pr848.osc-fr1.scalingo.io/health_checks/rdv_events_stats.json?min_mails=1 (statut http 416)