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/external_endpoint_test.go

@@ -0,0 +1,25 @@
+package core
+
+import (
+	"testing"
+)
+
+func TestExternalEndpoint_ToEndpoint(t *testing.T) {
+	externalEndpoint := &ExternalEndpoint{
+		Name:  "name",
+		Group: "group",
+	}
+	convertedEndpoint := externalEndpoint.ToEndpoint()
+	if externalEndpoint.Name != convertedEndpoint.Name {
+		t.Errorf("expected %s, got %s", externalEndpoint.Name, convertedEndpoint.Name)
+	}
+	if externalEndpoint.Group != convertedEndpoint.Group {
+		t.Errorf("expected %s, got %s", externalEndpoint.Group, convertedEndpoint.Group)
+	}
+	if externalEndpoint.Key() != convertedEndpoint.Key() {
+		t.Errorf("expected %s, got %s", externalEndpoint.Key(), convertedEndpoint.Key())
+	}
+	if externalEndpoint.DisplayName() != convertedEndpoint.DisplayName() {
+		t.Errorf("expected %s, got %s", externalEndpoint.DisplayName(), convertedEndpoint.DisplayName())
+	}
+}