This works the previous commits so that clients can add an addtional
'platform' query parameter to the webfinger request that can be used
to query the oidc client id and list of scopes that the clients need
to use when connecting to the IDP.
This also removes the non-standard issuer relatation introduced in a
previous commit as we can just introduce new relations in the
http://openid.net name space.
For IDP like Authentik that create a separate issuer url per Client
(Application in Authentik's terms) it is suggested to just configure
as single Client and use that id for all platforms (i.e. setting
'WEBFINGER_ANDROID_OIDC_CLIENT_ID', 'WEBFINGER_DESKTOP_OIDC_CLIENT_ID',
'WEBFINGER_IOS_OIDC_CLIENT_ID' and 'WEBFINGER_WEB_OIDC_CLIENT_ID' to
same value.
Related: #2088
Related: https://github.com/opencloud-eu/desktop/issues/246
