Get AAD service principal

• command: aad sp get -i|--appId [appId] -n|--displayName [displayName]
• appId: ID of the application for which the service principal should be retrieved
• displayName: display name of the application for which the service principal should be retrieved
• either appId or displayName must be specified but not both
• requires connection to AAD Graph

The information can be get from the following REST call:

GET https://graph.windows.net/myorganization/servicePrincipals?api-version=1.6&$filter=appId eq '<appId>'
authorization: Bearer eyJ0eXAi...

Requires #160 to be done first