endpoints: add Battlenet endpoints
endpoints: add Battlenet endpoints

OAuth APIs: https://develop.battle.net/documentation/battle-net/oauth-apis

Change-Id: I7c10f68c6ef0cb9b77cffa104e71a7198a599c89
GitHub-Last-Rev: e9596432c8cc6f91346cd6f38d14802ea6e35dd0
GitHub-Pull-Request: golang/oauth2#489
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/310249
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Trust: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Trust: Heschi Kreinick <e89ab2b172855aa0ad01c40c4d8a3535a61ac68a@google.com>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/oauth2