Add Managed Identity retry logic

[Avery-Dunn]

Implements retry logic for MI scenarios, as per https://github.com/AzureAD/microsoft-authentication-library-for-java/issues/690

The main changes are:

• Refactor HttpHelper to not have static methods, and it now extends a new IHttpHelper interface
• Add new HttpHelperManagedIdentity class that extends HttpHelper and implements MI-specific HTTP behavior (currently just the retry logic)
• Refactor ServiceBundle to take IHttpHelper as a parameter, so that the now non-static methods in HttpHelper can still be accessed

Issue https://github.com/AzureAD/microsoft-authentication-library-for-java/issues/690 also mentioned adding a way to disable this MI retry logic. Since MSAL Java doesn't have an API to disable the normal retry logic, there will be a different PR for the main library to add it

[neha-bhargava]

I would recommend adding unit tests that verify retry / not based on the returned status codes.