Add support for Azure AD - improvements

[ItalyPaleAle]

Sorry for the follow-up PR, I realized I could have improved on #33 a bit:

1. Use an interface for the SP provider object to avoid adding github.com/Azure/go-autorest/autorest/adal as a dependency to this module. So people who don't need to use Azure AD authentication don't need to import the entire package in their project
2. Fixed test that was leftover from before.

Please let me know when you think you can make a new release too :)

[a8m]

Thanks for your contribution @ItalyPaleAle. I actually prefer this approach.

Please let me know when you think you can make a new release too :)

I invited you as a collaborator for the project. Please let me know if you want to release this version and write a few notes about the new feature. Thanks 🙏

[ItalyPaleAle]

Thank you @a8m for merging!

I am happy to share some proposed release notes:

Added the ability to authenticate with Cosmos DB using Azure AD and enable using RBAC; this is optimized for usage with an *adal.ServicePrincipalToken object (#33, #34). See the Cosmos DB documentation for and the example for usage with this library.

Thanks for the invite to the project too. I don't think I can commit to being a maintainer, I'm just a user and contributing when I can :)