new port: security/rubygem-rack-oauth2

OAuth 2.0 Server & Client Library.

WWW: https://github.com/nov/rack-oauth2

PR: 199922
Differential Revision: https://reviews.freebsd.org/D2638
Submitted by: Torsten Zuehlsdorff <ports@toco-domains.de>
Approved by: mat (mentor)