Add frontend login support

Created by: schliesser

I would like to use this extension for backend and frontend logins with Azure AD. (The Azure AD Connection configuration is done in my project extension.) The purpose of this MR is to extend the login service also for the frontend.

First I extended the tx_oauth2_client_configs database table by the field parenttable to differentiate between frontend and backend users. The TCA is therefore also extended to fill this field automatically. (Maybe an upgrade wizard is needed to fill the field in existing projects?)

The field is also added to the fe_users TCA. To render the field for FE users I had to change the Oauth2ProvidersElement table check.

With the ModifyLoginFormViewListener the oauth2 providers are passed to the frontend login fluid template and the template is extended to render a button for each provider.

Now comes the tricky part 😉

After enabling the LoginService for the frontend it is now necessary to handle both login types. The BackendUserRepository is now extending the new UserRepository which allows to find frontend and backend users. The callbackUrl is also different for the frontend login plugin and therefore must be built in an other way.

The FrontendUserRepository is just a wrapper class which is not in use currently.

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I don't expect this to be merged as is 😉 But maybe this can be a base to add FE-Login support the extension. I think more steps are needed: e.g.: Each Provider can be enabled optionally for FE/BE.

Feel free to contact me via Slack or Discord.