Add support for Bullhorn API

[rguldener]

Api docs: https://bullhorn.github.io/rest-api-docs/#authorization

Unfortunately it is a non-standard flow. First there is an OAuth flow, but then the access token needs to be exchanged for a "login token". You authenticate API requests with this.
This acts like a session cookie that expires after a period of time. When that happens a new login token needs to be fetched.

There are also dynamic API base URLs that are returned with the login flow.

Details are here: https://bullhorn.github.io/Getting-Started-with-REST/