[feature] Command Payload Validation

[bounoable]

Support command payload validation:

package auth

type registerPayload struct {
	Name string
	Email string
}

func (payload registerPayload) Validate() error {
	if payload.Name == "" {
		return errors.New("empty name")
	}
	if !isValidEmail(payload.Email) {
		return fmt.Errorf("invalid email address: %s", payload.Email)
	}
	return nil
}

func RegisterUser(id uuid.UUID, name string, email string) command.Cmd[registerPayload] {
	return command.New("auth.user.register", registerPayload{name, email}, command.Aggregate("auth.user", id))
}

func HandleCommands(ctx context.Context, bus command.Bus) <-chan error {
	return command.MustHandle(ctx, bus, "auth.user.register", func(ctx command.Context) error {
		// ... this will only execute if ctx.Payload() valid ...
		return nil
	})
}