Add support for opaque AAD access tokens

[olegoid]

Consider the following set of scopes:
'profile', 'openid', 'offline_access', '9bd5ab7f-4031-4045-ace9-6bebbad202f6/all'

where return JSON looks somewhat like this:
https://gist.github.com/olegoid/7fec1889e98844686f84ae903a2e8d50

access_token claim parsing fails with Unable to read token claims error. For some resource scopes, AAD can issue so-called "opaque" tokens that are not supposed to be parsed. In that situation, we can rely on id_tokens to fetch supplemental information such as user name, etc.

Here's the document that describes that scenario:
https://docs.microsoft.com/en-us/azure/active-directory/develop/access-tokens

You can also try to manually validate the access_token from my JSON and it will turn out as gibberish. Although, the id_token is a totally valid JWT.

[kieferrm]

LGTM

@chrmarti pls review and merge if good.

[chrmarti]

Thanks!