Bug Fix: 'openid' added to scope even if already present

[mstrhakr]

This plugin adds the 'openid' scope to the request, even if that scope is already included, this can cause problems with some IdPs. (closes #98)

This pull fixes the problem by adding a check while building the params to see if 'openid' is already present in the scope, adding it if necessary.

This is tested and working with a combination of Meshcentral and Authelia