Implement picture resizer in Azure Function

[RobertoPrevato]

Currently the application only supports resizing pictures upon upload, using a thread pool handled by the API itself. This option is viable and should still be the default in Express mode.
This issue is about:

• adding also support for using an Azure Function (see Gallerist-AzureFunctions) (with ability to opt-in also in Express mode for this, similarly to the Application Insights integration)
• adding the Azure Function to the Bicep template
• adding the deployment to GitHub Workflows

Note: Gallerist-AzureFunctions must be upgraded before to the current version of Azure Functions for Python.