Adds oAuth2 badgr.io endpoints

[nqthqn]

Badgr.io is a service used to recognize and verify skills and achievements with the global standard in digital credentials; Open Badges empower people to take their learning with them, wherever they go, building a rich picture of their lifelong learning journey.

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This PR (HEAD: 3bbe099ca87ca6b7221498d8758975814a847e57) has been imported to Gerrit for code review.

Please visit https://go-review.googlesource.com/c/oauth2/+/362998 to see it.

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