Vendor omniauth-azure-oauth2 gem

What does this MR do and why?

• Our ultimate goal is to upgrade omniauth to v2.0. See the related issue
• In order to upgrade omniauth to v2.0, we need to upgrade omniauth-azure-oauth2 because the latter has a dependency on the former.
• There is an open issue asking the maintainer to make omniauth-azure-oauth2 compatible with omniauth v2.0 but the author advised forking the gem: https://github.com/marknadig/omniauth-azure-oauth2/pull/29#issuecomment-663113582
• There is a fork called omniauth-azure-activedirectory-v2 that supports only the 2.0 endpoints for Active Directory but we need to keep using omniauth-azure-oauth2 for our customers who use the 1.0 ActiveDirectory endpoints
• This MR vendors the gem and updates its .gemspec file so that it is compatible with omniauth v2.0
• We may deprecate our usage of this gem in the future (issue)
• MR where we vendored a different omniauth gem for same reasons: !92485 (merged)

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