Add managed identity support for auto-restart message pump on rotated credentials background job

[stijnmoreels]

Is your feature request related to a problem? Please describe.
Currently, we only support connection string support when registering an Azure Service Bus topic subscription that receives 'new secret version created' events to restart the message pump.

Describe the solution you'd like
Add support for managed identity with the AddCloudEventBackgroundJobUsingManagedIdentity overload. Make sure to test this with our TemporaryEnvironmentVariable so that the secret values are retrieved from the environment.
Take a look at how the AddCloudEventBackgroundJobUsingManagedIdentity is tested for an example.