Resolve "keycloak config als eigene Klasse einführen"

What is new and why?

• RF: eigene Authentication-Klasse für Keycloak-Settings
• BF: setze guest_mode in tests/conftest.py explizit
• RF: idp_openid_configuration wurde zu openid_configuration_url
• RF: openid_configuration_url ist kein User-Input mehr, dafür aber keycloak_base_url und keycloak_realm
• ENH: get_current_user() wird jetzt mit await
• NF: neuer Service, der nur prüft ob Keycloak bereit ist, damit Core erst danach startet. Keycloak selbst kann das scheinbar nicht mitteilen

Related issues

Link related issues below.

Author's TODOs

• if the licence_check-stage is not green: Download artefact (view in browser does not work for now) and check findings.json
• merged latest version of target branch (e.g., main or dev) into your branch
• check if SPDX identifier MPL-2.0 and Copyright-Text 2025 Land Baden-Württemberg <InnoLab@stm.bwl.de> exists in newly created and/or hasn't been deleted somehow in existing files
• adapted the readme
• Revised/new code parts have no # noqa directives
• Optional: Changelog adapted (further information)
• Optional: Update OpenAPI documentation whenever endpoints, API input classes, or API output classes change (see details)

Reviewer's TODOs

• Check if the Author did their checks well enough
• Tried out if the cause of the issue is solved as intended
• Check that code with old # noqa directives were revised
• Check that no new # noqa directives are included
• Optional: Changelog adapted (further information)

To keep in mind

• pre-commit used
• added logging output were necessary
• added/adjusted Errorhandling to avoid unwanted crashes and make proper debugging and testing possible
• Changelog has to be adapted before merging, at latest in the f13::inAbnahme step

Closes #35 (closed)