Feature: drop dependency on username and password for connector configuration

[donatorsky]

Feature description

For connector to work, it essentially needs Client ID, Client Secret and an OAuth Access Token, so Username, Password and Security Token can potentially be dropped. However, this requires a mechanism to authorize the user from the outside of the connector and pass the response to it.

With this issue, there is another thing to discuss. OAuth's tokens usually have a fixed expiration date. Without credentials and Refresh Token, connector cannot request for Access Token by itself while working and has to fail gracefully.