feat(alerting): Implement alert reminders (#1138)

* feat(alerting): add reminder-interval feature which allows setting an interval to run alert consecutively

* feat(test): add tests for reminder-interval feature

* feat(docs): modify documentation for reminder-interval feature

* chore: change "due" to "TRIGGERED" for easier log look through

* chore: update "reminder-interval" to "repeat-interval"

* chore: update reminder-interval to repeat-interval

* chore: adapt repeat interval feature after merge

* chore: adapt repeat interval feature after merge

* RepeatInterval => MinimumRepeatInterval

* fix merge issues

(cherry picked from commit 9b2161556bddf01d385f97dafac2515857190ae5)

* rename and move MiniumRepeatInterval

* move MiniumRepeatInterval (again)

---------

Co-authored-by: Bugra Kocabay <kocabay.bugra@gmail.com>
Co-authored-by: Bugra Kocabay <kocabaybugra@gmail.com>
Co-authored-by: Konstantin Nosov <nosovk@gmail.com>
Co-authored-by: Viktor Ziegler <Viktor.Ziegler@ti8m.ch>
Co-authored-by: TwiN <twin@linux.com>

config/endpoint/endpoint.go

@@ -131,6 +131,9 @@ type Endpoint struct {
 
 	// NumberOfSuccessesInARow is the number of successful evaluations in a row
 	NumberOfSuccessesInARow int `yaml:"-"`
+
+	// LastReminderSent is the time at which the last reminder was sent for this endpoint.
+	LastReminderSent time.Time `yaml:"-"`
 }
 
 // IsEnabled returns whether the endpoint is enabled or not