Add the ability to use personal access token authentication at Github plugin

As per Deprecated APIs and authentication - GitHub Developer Guide, GitHub will shortly deprecate password authentication to their API.

As Bamboo uses this method of authentication to get information about the repositories via API, the plugin should be adjusted so personal access token can be used while adding new GitHub repositories to Bamboo.

I.e

Requests on the following API endpoint that is currently used by Bamboo will stop working:

https://api.github.com/user/repos?per_page=100

All password authentication will return a status code of 401 starting:

• November 13, 2020 at 4:00 PM UTC