feat(auth): allow setting external secure

[muhlemmer]

This change adds the possibility to set an external secure flag to the Authenticator. This allows creating logout URLs with https protocol when the service is behind a reverse proxy.

Closes #285

Definition of Ready

• [x] I am happy with the code
• [x] Short description of the feature/issue is added in the pr description
• [x] PR is linked to the corresponding user story
• [x] Acceptance criteria are met
• [x] All open todos and follow ups are defined in a new ticket and justified
• [x] Deviations from the acceptance criteria and design are agreed with the PO and documented.
• [x] No debug or dead code
• [x] My code has no repetitions
• [ ] Critical parts are tested automatically
• [ ] Where possible E2E tests are implemented
• [x] Documentation/examples are up-to-date
• [x] All non-functional requirements are met
• [ ] Functionality of the acceptance criteria is checked manually on the dev system.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 0.05%. Comparing base (bfb0be0) to head (53cb67a). Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/authentication/authenticate.go	0.00%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #389 +/- ## ===================================== Coverage 0.05% 0.05% ===================================== Files 127 127 Lines 92980 92983 +3 ===================================== Hits 47 47 - Misses 92930 92933 +3 Partials 3 3 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[github-actions[bot]]

:tada: This PR is included in version 3.2.0 :tada:

The release is available on GitHub release

Your semantic-release bot :package::rocket: