Add client_secret to oidc requests

[fm3]

Steps to test:

• Set up keycloak via docker-compose as described in Add OIDC authentication #6534 (comment) (Note that the admin user in keycloak needs first name, last name, email, which was missing by default when I tried this)
• Adapt the client config in the keycloak UI to use Client authentication. A new tab “credentials” appears. Copy the client secret and add it in the application.conf
• Also double-check the other values, especially publicKey, take that from Realm Settings → Keys in the keycloak UI
• Also set features.openIdConnectEnabled = true in application.conf
• Sign up/log in in webknossos using the OIDC button
• Should work if client secret is correct, not work otherwise
• Should also work if client secret is disabled in keycloak (set to emptystring in application.conf)

Issues:

• fixes Add OIDC client_secret-based authorization #7260

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• Updated changelog

[normanrz]

Nice stuff!