Migrate deployment of UserNotifications service to Bicep

[JamesDawson]

A spike has been carried-out in Marain.Tenancy which will have been productionised as part of marain-dotnet/Marain.Tenancy#396.

This issue relates to applying those same principles to the service in this repository and replacing the existing deployment scripts.

Including:

1. Support for hosting in Azure Functions & Container Apps
2. Simplified configuration management through use of Azure AppConfiguration
3. Integration with source-controlled deployment configuration work planned here:
   • Add centralised deployment configuration store Marain.Deployment.Template#4