feat: Let the slack API URL be set (#123)

defaults.go

@@ -5,6 +5,13 @@ import "github.com/slack-go/slack"
 // ClientOption an option for client values
 type ClientOption func(*ClientDefaults)
 
+// WithAPIURL sets the API URL (for testing)
+func WithAPIURL(url string) ClientOption {
+	return func(defaults *ClientDefaults) {
+		defaults.APIURL = url
+	}
+}
+
 // WithDebug sets debug toggle
 func WithDebug(debug bool) ClientOption {
 	return func(defaults *ClientDefaults) {
@@ -21,12 +28,14 @@ func WithBotInteractionMode(mode BotInteractionMode) ClientOption {
 
 // ClientDefaults configuration
 type ClientDefaults struct {
+	APIURL  string
 	Debug   bool
 	BotMode BotInteractionMode
 }
 
 func newClientDefaults(options ...ClientOption) *ClientDefaults {
 	config := &ClientDefaults{
+		APIURL:  "", // Empty string will not override default from slack package
 		Debug:   false,
 		BotMode: BotInteractionModeIgnoreAll,
 	}

slacker.go

@@ -42,10 +42,18 @@ func defaultCleanEventInput(msg string) string {
 func NewClient(botToken, appToken string, options ...ClientOption) *Slacker {
 	defaults := newClientDefaults(options...)
 
-	api := slack.New(
-		botToken,
+	slackOpts := []slack.Option{
 		slack.OptionDebug(defaults.Debug),
 		slack.OptionAppLevelToken(appToken),
+	}
+
+	if defaults.APIURL != "" {
+		slackOpts = append(slackOpts, slack.OptionAPIURL(defaults.APIURL))
+	}
+
+	api := slack.New(
+		botToken,
+		slackOpts...,
 	)
 
 	socketModeClient := socketmode.New(