Include team ID in Slack OIDC response

[trobinpl]

Is your feature request related to a problem? Please describe.

Due to the business requirements in my multi-tenant application I need to make sure that when user clicks "Sign in with Slack" he has indeed an account belonging to the team that was previously connected with the tenant this user is trying to log into. Since I haven't found any way of enforcing it during the redirect-to-Slack phase I thought it can be validated in the post-registration hook.

The slack-go package used inside Slack provider seems to include team ID in it's response (it is definitely included in the Slack response itself).
I'm happy to provide such PR I've went ahead and already created the PR #1409 , but wanted to verify if that is indeed the best solution to this problem.

Describe the solution you'd like

The desired change should be as small as adding

Team: identity.Team.ID,

after this line

Describe alternatives you've considered

I was exploring a possibility to add &team param at the end of oauth/authorize request which seems to work fine with the v2 version of this endpoint