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_test.go

@@ -0,0 +1,51 @@
+package core
+
+import (
+	"errors"
+	"testing"
+
+	"github.com/TwiN/gatus/v5/alerting/alert"
+)
+
+func TestValidateEndpointNameGroupAndAlerts(t *testing.T) {
+	scenarios := []struct {
+		name        string
+		group       string
+		alerts      []*alert.Alert
+		expectedErr error
+	}{
+		{
+			name:   "n",
+			group:  "g",
+			alerts: []*alert.Alert{{Type: "slack"}},
+		},
+		{
+			name:   "n",
+			alerts: []*alert.Alert{{Type: "slack"}},
+		},
+		{
+			group:       "g",
+			alerts:      []*alert.Alert{{Type: "slack"}},
+			expectedErr: ErrEndpointWithNoName,
+		},
+		{
+			name:        "\"",
+			alerts:      []*alert.Alert{{Type: "slack"}},
+			expectedErr: ErrEndpointWithInvalidNameOrGroup,
+		},
+		{
+			name:        "n",
+			group:       "\\",
+			alerts:      []*alert.Alert{{Type: "slack"}},
+			expectedErr: ErrEndpointWithInvalidNameOrGroup,
+		},
+	}
+	for _, scenario := range scenarios {
+		t.Run(scenario.name, func(t *testing.T) {
+			err := validateEndpointNameGroupAndAlerts(scenario.name, scenario.group, scenario.alerts)
+			if !errors.Is(err, scenario.expectedErr) {
+				t.Errorf("expected error to be %v but got %v", scenario.expectedErr, err)
+			}
+		})
+	}
+}