feat(alerting): custom alert support endpoint errors (#844)

* feat(alerting): add support for including endpoint errors in custom alerts

- Updated `buildHTTPRequest` method in `AlertProvider` to accept a `result` parameter.
- Added support for including `[ENDPOINT_ERRORS]` in both the request body and URL, which will be replaced by the errors from `Result.Errors[]`.
- Adjusted `CreateExternalEndpointResult` to capture and store errors from query parameters.
- This allows custom alerts to include detailed error information, enhancing the flexibility of alert notifications.

* feat: add ENDPOINT_ERRORS example

* feat: add tests

* Refactor: code review feedback

* delete unsed errors

* Update README.md

* Apply suggestions from code review

---------

Co-authored-by: raojinlin <raojinlin302@gmail.com>
Co-authored-by: TwiN <twin@linux.com>

api/external_endpoint.go

@@ -46,6 +46,9 @@ func CreateExternalEndpointResult(cfg *config.Config) fiber.Handler {
 			Success:   c.QueryBool("success"),
 			Errors:    []string{},
 		}
+		if !result.Success && c.Query("error") != "" {
+			result.Errors = append(result.Errors, c.Query("error"))
+		}
 		convertedEndpoint := externalEndpoint.ToEndpoint()
 		if err := store.Get().Insert(convertedEndpoint, result); err != nil {
 			if errors.Is(err, common.ErrEndpointNotFound) {