feat(alerting): Add Pushover provider (#405)

* Add a new Pushover provider (#129)
- Adds new provider named Pushover with corresponding tests
- Adds Pushover as a provider to the configuration and adjusts test accordingly
- Adds Pushover to alerting_test.go, provider.go and type.go
- Updates the readme with configuration details

* Correct import order

* Fix some missing pushover references

* Apply suggestions from code review

* Rename application-key to application-token for Pushover

---------

Co-authored-by: TwiN <twin@linux.com>

alerting/provider/pushover/pushover.go

@@ -0,0 +1,106 @@
+package pushover
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+
+	"github.com/TwiN/gatus/v5/alerting/alert"
+	"github.com/TwiN/gatus/v5/client"
+	"github.com/TwiN/gatus/v5/core"
+)
+
+const (
+	restAPIURL      = "https://api.pushover.net/1/messages.json"
+	defaultPriority = 0
+)
+
+// AlertProvider is the configuration necessary for sending an alert using Pushover
+type AlertProvider struct {
+	// Key used to authenticate the application sending
+	// See "Your Applications" on the dashboard, or add a new one: https://pushover.net/apps/build
+	ApplicationToken string `yaml:"application-token"`
+
+	// Key of the user or group the messages should be sent to
+	UserKey string `yaml:"user-key"`
+
+	// The title of your message, likely the application name
+	// default: the name of your application in Pushover
+	Title string `yaml:"title,omitempty"`
+
+	// Priority of all messages, ranging from -2 (very low) to 2 (Emergency)
+	// default: 0
+	Priority int `yaml:"priority,omitempty"`
+
+	// DefaultAlert is the default alert configuration to use for endpoints with an alert of the appropriate type
+	DefaultAlert *alert.Alert `yaml:"default-alert,omitempty"`
+}
+
+// IsValid returns whether the provider's configuration is valid
+func (provider *AlertProvider) IsValid() bool {
+	if provider.Priority == 0 {
+		provider.Priority = defaultPriority
+	}
+	return len(provider.ApplicationToken) == 30 && len(provider.UserKey) == 30 && provider.Priority >= -2 && provider.Priority <= 2
+}
+
+// Send an alert using the provider
+// Reference doc for pushover: https://pushover.net/api
+func (provider *AlertProvider) Send(endpoint *core.Endpoint, alert *alert.Alert, result *core.Result, resolved bool) error {
+	buffer := bytes.NewBuffer(provider.buildRequestBody(endpoint, alert, result, resolved))
+	request, err := http.NewRequest(http.MethodPost, restAPIURL, buffer)
+	if err != nil {
+		return err
+	}
+	request.Header.Set("Content-Type", "application/json")
+	response, err := client.GetHTTPClient(nil).Do(request)
+	if err != nil {
+		return err
+	}
+	defer response.Body.Close()
+	if response.StatusCode > 399 {
+		body, _ := io.ReadAll(response.Body)
+		return fmt.Errorf("call to provider alert returned status code %d: %s", response.StatusCode, string(body))
+	}
+	return err
+}
+
+type Body struct {
+	Token    string `json:"token"`
+	User     string `json:"user"`
+	Title    string `json:"title,omitempty"`
+	Message  string `json:"message"`
+	Priority int    `json:"priority"`
+}
+
+// buildRequestBody builds the request body for the provider
+func (provider *AlertProvider) buildRequestBody(endpoint *core.Endpoint, alert *alert.Alert, result *core.Result, resolved bool) []byte {
+	var message string
+	if resolved {
+		message = fmt.Sprintf("RESOLVED: %s - %s", endpoint.DisplayName(), alert.GetDescription())
+	} else {
+		message = fmt.Sprintf("TRIGGERED: %s - %s", endpoint.DisplayName(), alert.GetDescription())
+	}
+	body, _ := json.Marshal(Body{
+		Token:    provider.ApplicationToken,
+		User:     provider.UserKey,
+		Title:    provider.Title,
+		Message:  message,
+		Priority: provider.priority(),
+	})
+	return body
+}
+
+func (provider *AlertProvider) priority() int {
+	if provider.Priority == 0 {
+		return defaultPriority
+	}
+	return provider.Priority
+}
+
+// GetDefaultAlert returns the provider's default alert configuration
+func (provider AlertProvider) GetDefaultAlert() *alert.Alert {
+	return provider.DefaultAlert
+}