Allow configuration of redirectURL

Currently the Authenticator extension automatically gets the redirect URL based on the address of the XWiki instance. In some cases it may be necessary to specify a different redirect URL.

In our case we use XWiki in Docker with a reverse proxy in the same Docker network. SSL/TLS is terminated at the proxy and communication between proxy and XWiki is done over port 8080 (No changes to the default tomcat configuration). When authentication is performed the automatically generated redirect URL is https://the.xwiki.instance:80/oidc/authenticator/callback which causes an error since port 80 does not use SSL/TLS. When using https://the.xwiki.instance/oidc/authenticator/callback as the redirect URL everything works fine.  (This use case was a configuration issue.)

To achieve this we added the new property oidc.redirecturl to the configuration manager. If it is set this redirect URL will be used. If it is not set or empty the URL will be fetched automatically as it was before.