Work on common provider interface to make adding new providers easier

alerting/provider/custom/custom.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"fmt"
 	"github.com/TwinProduction/gatus/client"
+	"github.com/TwinProduction/gatus/core"
 	"io/ioutil"
 	"net/http"
 	"strings"
@@ -20,6 +21,10 @@ func (provider *AlertProvider) IsValid() bool {
 	return len(provider.Url) > 0
 }
 
+func (provider *AlertProvider) ToCustomAlertProvider(service *core.Service, alert *core.Alert, result *core.Result, resolved bool) *AlertProvider {
+	return provider
+}
+
 func (provider *AlertProvider) buildRequest(serviceName, alertDescription string, resolved bool) *http.Request {
 	body := provider.Body
 	providerUrl := provider.Url