Add "provider" property to user object

[joepavitt]

Currently speaking with a community member (@fullmetal-fred) who has developed an auth plugin for Cloudflare, uses the same plugin/socketio approach as us. Has asked some questions about packaging, clashing, etc. one thing he and I have agreed on, is that there is merit in adding a _provider property to the user object so we know which plugin has populated the relevant user object