feat: Implement push-based external endpoints (#724)

* refactor: Move SSH outside of endpoint.go
* refactor: Use pointers for Alert receivers
* feat: Implement push-based external endpoints
* Fix failing tests
* Validate external endpoints on start
* Add tests for external endpoints
* refactor some error equality checks
* Improve docs and refactor some code
* Fix UI-related issues with external endpoints

core/endpoint_common.go

@@ -0,0 +1,32 @@
+package core
+
+import (
+	"errors"
+	"strings"
+
+	"github.com/TwiN/gatus/v5/alerting/alert"
+)
+
+var (
+	// ErrEndpointWithNoName is the error with which Gatus will panic if an endpoint is configured with no name
+	ErrEndpointWithNoName = errors.New("you must specify a name for each endpoint")
+
+	// ErrEndpointWithInvalidNameOrGroup is the error with which Gatus will panic if an endpoint has an invalid character where it shouldn't
+	ErrEndpointWithInvalidNameOrGroup = errors.New("endpoint name and group must not have \" or \\")
+)
+
+// validateEndpointNameGroupAndAlerts validates the name, group and alerts of an endpoint
+func validateEndpointNameGroupAndAlerts(name, group string, alerts []*alert.Alert) error {
+	if len(name) == 0 {
+		return ErrEndpointWithNoName
+	}
+	if strings.ContainsAny(name, "\"\\") || strings.ContainsAny(group, "\"\\") {
+		return ErrEndpointWithInvalidNameOrGroup
+	}
+	for _, endpointAlert := range alerts {
+		if err := endpointAlert.ValidateAndSetDefaults(); err != nil {
+			return err
+		}
+	}
+	return nil
+}