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.go

@@ -0,0 +1,29 @@
+package core
+
+import (
+	"errors"
+)
+
+var (
+	// ErrEndpointWithoutSSHUsername is the error with which Gatus will panic if an endpoint with SSH monitoring is configured without a user.
+	ErrEndpointWithoutSSHUsername = errors.New("you must specify a username for each SSH endpoint")
+
+	// ErrEndpointWithoutSSHPassword is the error with which Gatus will panic if an endpoint with SSH monitoring is configured without a password.
+	ErrEndpointWithoutSSHPassword = errors.New("you must specify a password for each SSH endpoint")
+)
+
+type SSH struct {
+	Username string `yaml:"username,omitempty"`
+	Password string `yaml:"password,omitempty"`
+}
+
+// validate validates the endpoint
+func (s *SSH) validate() error {
+	if len(s.Username) == 0 {
+		return ErrEndpointWithoutSSHUsername
+	}
+	if len(s.Password) == 0 {
+		return ErrEndpointWithoutSSHPassword
+	}
+	return nil
+}