Vendor omniauth-salesforce gem

* Our ultimate goal is to upgrade `omniauth` to v2.0. See the related
  issue: https://gitlab.com/gitlab-org/gitlab/-/issues/30073
* In order to upgrade `omniauth` to v2.0, we need to upgrade
  `omniauth-salesforce` because the latter has a dependency on the former.
* There is an open issue asking the maintainer to make
  `omniauth-salesforce` compatible with `omniauth` v2.0 but that hasn't
  happened yet: https://github.com/realdoug/omniauth-salesforce/issues/31
* We are actively emailing with the gem owner about transferring or
  adding additional contributors to the gem but in the meantime this
  will unblock the upgrade.
* This MR vendors the gem and updates its `.gemspec` file so that it is
  compatible with `omniauth` v2.0