Support Slack V2 endpoints #798
Description
Summary of proposed feature
The current implementation of Slack is for legacy Slack apps
Purpose of proposed feature
New Slack Apps must use the newer oauth.v2.access endpoint. I'd like next-auth to support that
Detail about proposed feature
Slack now has new OAuth endpoints for new Slack Apps
AuthURL: "https://slack.com/oauth/v2/authorize",
TokenURL: "https://slack.com/api/oauth.v2.access",
