Mac OS X - error during installation of Summa software (Malicious software)

When installing Summa Plug-in on latest Mac OS X systems an error could pop-up showing the software can not be installed as it is "Malicious software":

• "summa-goproduce-illustrator-plug-in-for-macos.pkg" can't be opened because Apple cannot check it for malicious software.
• "summa-gosign-illustrator-2021-24-plug-in-for-macos.pkg" can't be opened because Apple cannot check it for malicious software. 
• "macsign-plug-in-2-1-1-for-ai-2021-2024.pkg" can't be opened because Apple cannot check it for malicious software. 

The problem encountered is common to all software downloaded from the internet and NOT coming from the official Apple Store.

With the latest release of OSX to execute a software NOT downloaded from Apple Store (in this case downloaded from Summa web site) the user must:

Select the installer and choose “the File->Open” command by the Finder

OR

Select the installer and click with the right button of the mouse 

in both these cases an Open button is added to the dialog, the Open button is not shown with a simple double click on the installer icon (only the Cancel one)