Hello Giuseppe C,
Thanks for your question.
As of now, the Azure Functions Flex Consumption plan does not support custom handlers. The available runtime stacks include Node.js, PowerShell, Python, Java, and .NET.
I could not find any current update on plans to integrate this feature in my search for you. However you can track current changes and updates here: https://azure.microsoft.com/en-us/updates/
If it is a feature you need you can also post https://feedbackportal.microsoft.com/feedback/ as engineers usually monitor this channel to see most upvoted features and possibilities of integration.
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola