Refactor OmniauthCallbacksController to remove duplication

What does this MR do?

Begin refactoring OmniauthCallbacksController.

• Creates specific controller for LDAP, instead of duplicating methods with an ldap_ prefix.
• Adds LDAP controller tests.
• Extracts IdentityLinker object to handle code paths which differ between saml and handle_omniauth.

Why was this MR needed?

Much duplication.

I'll be adding a new :group_saml provider with different logic in https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4076 and didn't want to duplicate things yet again.

Acceptance criteria

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend
• End-to-end tests pass (package-qa manual pipeline job)

Related

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/26559 with 6ab92416