* Added endpoint and backing service for backoffice login providers and the status per user.
* Improve link login redirect forming and error handling
* Add responseModel and mapping instead of returning core model
* Moved unlink endpoint logic into a service
* Refactored ExternalLinkLoginCallback logic into BackofficeExternalLoginService method
* typo and minor code style improvements
* async method name alignment
* Add BackOfficeExternalLoginService tests
* Remove helper method that makes less sense that thought.
* Minor formatting, clean-up and conventions
* Replaced cookie authentication in link-login with a short lived secret
Applied PR feedback
* Update openapi
* Changed link login to a form endpoint
* fix broken comment link
* Do not store claimsprinciple in secret + comments
* update redirect paths
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Co-authored-by: Sven Geusens <sge@umbraco.dk>
Co-authored-by: kjac <kja@umbraco.dk>
Co-authored-by: Jacob Overgaard <752371+iOvergaard@users.noreply.github.com>
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