fix: add flag for auth in keycloak realm url

[rekt-hard]

:heart: Thank you for your contribution!

Description

closes #323

Add flag for using the auth/ part in the realm url. Newer versions of keycloak do not have an auth/ part in the realm url per default anymore. To not break existing InvenioRDM instances and oauth configurations, the flag is set to true per default.

This allows to keep all the other setup functionality and only modify the realm url if needed. Keycloak v16 support has been deprecated for quite some time now ((11 Mar 2022) according to https://endoflife.date/keycloak).

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[max-moser]

thanks!

[ntarocco]

Needs release?

[rekt-hard]

Will be addressing the comment in https://github.com/inveniosoftware/invenio-oauthclient/pull/326 A release after would be great

Or two small releases. However you prefer