fn: add ContextGuard.Goroutine method

Add the ContextGuard.Goroutine method for spawning goroutines within the
context guard's scope. This method manages the wait group and ensures
consistent error handling for the spawned goroutine via a provided
error handler callback.

We choose the longer name `Goroutine` rather than `Go` to draw attention
to the fact that this function starts a new goroutine.

Author: ffranr

fn/context_guard.go

@@ -99,3 +99,29 @@ func (g *ContextGuard) WithCtxQuitNoTimeout() (context.Context, func()) {
 
 	return ctx, cancel
 }
+
+// Goroutine runs the given function in a separate goroutine and ensures proper
+// error handling. If the object function returns an error, the provided error
+// handler is called.
+//
+// This method also manages the context guard wait group when spawning the
+// goroutine.
+func (g *ContextGuard) Goroutine(f func() error, errHandler func(error)) {
+	if f == nil {
+		panic("no function provided")
+	}
+
+	if errHandler == nil {
+		panic("no error handler provided")
+	}
+
+	g.Wg.Add(1)
+	go func() {
+		defer g.Wg.Done()
+
+		err := f()
+		if err != nil {
+			errHandler(err)
+		}
+	}()
+}