[8.0] IdP clients per user group

[atsareg]

This PR adds the possibility to define IdP client credentials per user group to allow using the same IdProvider for several VOs without the need to define separate IdProvider per VO. If no setting is added for a group, generic client credentials will be taken. So, existing IdP configurations will be still valid. Example IdP configuration:

  Resources
  {
    IdProviders
    {
      <IdProvider name>
      {
        client_id = <client_id>
        client_secret = <client_secret>
        dirac_pilot
        {
          client_id = <client_id_for_dirac_VO>
          client_secret = <client_secret_for_dirac_VO>
        }
        biomed_pilot
        {
          client_id = <client_id_for_biomed_VO>
          client_secret = <client_secret_for_biomed_VO>
        }
      }
    }
  }

BEGINRELEASENOTES

*Framework
NEW: Possibility to define several OAuth2 clients for a given IdP for different DIRAC groups

ENDRELEASENOTES

[aldbr]

LGTM

[chaen]

For my education, what is the gain of doing it like this instead of using one IdP per vo ?
Also, given the examples you write, it is aimed at the pilot submission. But how does it work when you need tkens for other purposes which use a different group (VOMS2CS, FTS3, etc) ?