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/ssh_test.go

@@ -0,0 +1,25 @@
+package core
+
+import (
+	"errors"
+	"testing"
+)
+
+func TestSSH_validate(t *testing.T) {
+	ssh := &SSH{}
+	if err := ssh.validate(); err == nil {
+		t.Error("expected an error")
+	} else if !errors.Is(err, ErrEndpointWithoutSSHUsername) {
+		t.Errorf("expected error to be '%v', got '%v'", ErrEndpointWithoutSSHUsername, err)
+	}
+	ssh.Username = "username"
+	if err := ssh.validate(); err == nil {
+		t.Error("expected an error")
+	} else if !errors.Is(err, ErrEndpointWithoutSSHPassword) {
+		t.Errorf("expected error to be '%v', got '%v'", ErrEndpointWithoutSSHPassword, err)
+	}
+	ssh.Password = "password"
+	if err := ssh.validate(); err != nil {
+		t.Errorf("expected no error, got '%v'", err)
+	}
+}