New(?) undocumented method to resend an invitation

[manuelmeurer]

I used to use users_admin_invite with the "resend" param to resend an invitation to a user when the original invitation was expired.
This seems to trigger the "already_in_team_invited_user" error now.

From watching the requests when resending an invite via the https://myworkspace.slack.com/admin/invites I noticed a new(?) endpoint "users.admin.resendInvitation":

URL: https://myworkspace.slack.com/api/users.admin.resendInvitation
Form Data:
token: xoxc-...
invite_id: 1234
_x_reason: admin-invite-actions
_x_mode: online

Can we add this to lib/slack/web/api/endpoints/users_admin.rb ?

[dblock]

Start in https://github.com/slack-ruby/slack-api-ref. We scrape Slack docs - if the API is not mentioned in those docs you can add it manually to the ref as undocumented. Then it will come in via #578.