adding support for managed identity connections to underlying host st…

[SebastianClaesson]

• Added the appSettings as a Microsoft.Web/sites/config.
• Changed the additionalAppSettings to an object type instead of array
• FUNCTIONS_APP_EDIT_MODE set to readonly
• StorageQueueConnection__credential set to managedidentity
• WEBSITE_CONTENTAZUREFILECONNECTIONSTRING references a Azure Key vault secret instead
• Always creating an Azure keyvault as part of the solution, to keep sensitive information.
• Added role assignments for the managed identity to the underlying host storage account

To assist with #674

[shibayan]

As I commented in the Issue, I believe it is too complex to incorporate into the default deployment template.