Missing integration tests for Azure Key vault secret provider with service principal certificate authentication

[stijnmoreels]

Describe the problem.
Due to the highly supported variations of the Key vault secret provider, the service principal certificate authentication was overlooked. An added integration test would actually mostly test the Microsoft-related certificate authentication, but it is good to investigate in whether all the Arcus parts are working together, rather than a valid certificate authentication.
Nonetheless, we should look into providing some integration tests for this.