ADAL is deprecated

[benvdh]

Hi Chris,

While working on my project based on pypowerbi, I noticed ADAL has been deprecated for a while now. So I'm using the new MSAL library instead. One of the main tricky parts I noticed is that when passing the authentication token, that ADAL apparently used snake_case while MSAL uses CamelCase for its dictionary keys, this caused some exceptions being thrown by pypowerbi not finding the expected key (i.e. auth_token v.s. authToken)...

I'm just adding the issue here, to not forget about it. I can create a pull request for initial MSAL support.

One question, though, @cmberryau : should we maintain support in pypowerbi for ADAL, or can we drop support for it completely?

Announcement:
https://github.com/AzureAD/azure-activedirectory-library-for-python

Migration guide:
https://docs.microsoft.com/en-us/azure/active-directory/develop/migrate-python-adal-msal