Refactoring Gitlab::Geo::OauthSession

What does this MR do?

The Gitlab::Geo::OauthSession has a lot of responsibilities, this MR extracts each responsability into a proper class:

• GitLab::Geo::Oauth::Session
• GitLab::Geo::Oauth::LoginState
• GitLab::Geo::Oauth::LogoutState
• GitLab::Geo::Oauth::LogoutToken
• GitLab::ReturnToLocation

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/8422

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the [Test Planning Process](https://about.gitlab.com/handbook/engineering/quality/test-engineering/).
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?