Enables Client Credentials to call Microsoft Graph

[roryclaasen]

There is a setting in Microsoft Azure Active Directory that prevents guest users from retrieving graph data. Including the group display name, which is used in group synchronization.

https://docs.microsoft.com/en-us/azure/active-directory/users-groups-roles/users-restrict-guest-permissions

With this setting set to restricted, the graph api returns empty group objects and therefore doesn't sync groups.

This PR adds a new option to allow the use of client credentials. So instead of granting an application Directory.AccessAsUser.All (Delegated) you need to give it Directory.Read.All (Application).
Directory.Read.All is the least most privileged permission required as per https://docs.microsoft.com/en-us/graph/api/user-list-transitivememberof.

As the application permissions are granted for the tenant, this setting cannot be used with multi tenant enabled.

The initial authContext.acquireTokenByAuthorizationCode is still required to retrieve the user information from the id token claims, as authContext.acquireToken wont retrieve this data.

[roryclaasen]

@hkamel @srvrguy Bumping this PR

My team would rather use an official build rather than a local built of the plugin and so if this PR could get feedback/merged that'll be great