Initial re authentication implementation

Introduces the StreamingCredentialsProvider as the CredentialsProvider
with the highest priority.

TODO: needs to be tested

auth/auth.go

@@ -9,6 +9,7 @@ type StreamingCredentialsProvider interface {
 	// Subscribe subscribes to the credentials provider for updates.
 	// It returns the current credentials, a cancel function to unsubscribe from the provider,
 	// and an error if any.
+	// TODO(ndyakov): Should we add context to the Subscribe method?
 	Subscribe(listener CredentialsListener) (Credentials, CancelProviderFunc, error)
 }
 

auth/reauth_credentials_listener.go

@@ -0,0 +1,45 @@
+package auth
+
+// ReAuthCredentialsListener is a struct that implements the CredentialsListener interface.
+// It is used to re-authenticate the credentials when they are updated.
+// It contains:
+// - reAuth: a function that takes the new credentials and returns an error if any.
+// - onErr: a function that takes an error and handles it.
+type ReAuthCredentialsListener struct {
+	reAuth func(credentials Credentials) error
+	onErr  func(err error)
+}
+
+// OnNext is called when the credentials are updated.
+// It calls the reAuth function with the new credentials.
+// If the reAuth function returns an error, it calls the onErr function with the error.
+func (c *ReAuthCredentialsListener) OnNext(credentials Credentials) {
+	if c.reAuth != nil {
+		err := c.reAuth(credentials)
+		if err != nil {
+			if c.onErr != nil {
+				c.onErr(err)
+			}
+		}
+	}
+}
+
+// OnError is called when an error occurs.
+// It can be called from both the credentials provider and the reAuth function.
+func (c *ReAuthCredentialsListener) OnError(err error) {
+	if c.onErr != nil {
+		c.onErr(err)
+	}
+}
+
+// NewReAuthCredentialsListener creates a new ReAuthCredentialsListener.
+// Implements the auth.CredentialsListener interface.
+func NewReAuthCredentialsListener(reAuth func(credentials Credentials) error, onErr func(err error)) *ReAuthCredentialsListener {
+	return &ReAuthCredentialsListener{
+		reAuth: reAuth,
+		onErr:  onErr,
+	}
+}
+
+// Ensure ReAuthCredentialsListener implements the CredentialsListener interface.
+var _ CredentialsListener = (*ReAuthCredentialsListener)(nil)