add extra_config for openid clients to handle custom attributes (#387)

[olivierboudet]

Hello @mrparkers ,

As the PR #389 seems blocked since several months and because I need to manage custom attributes on openid clients, I tried to make a new PR. This new PR handles only a new extra_config field on keycloak_openid_client.

However I have a question : as Keycloak recently added 2 new attributes with default values (backchannel.logout.revoke.offline.tokens=false and backchannel.logout.session.required=true, as described in #568), how should we handle these attributes ? I modified all the tests to add these new attributes in data, is this correct waiting for an official support for these fields ?
The advantage on this solution is that it allows to set all new attributes added by Keycloak before an official support in the provider.
The drawback is it become mandatory to set this extra_config when using KC >= 12.x with a provider which do not support this fields natively.

extra_config = {
  "backchannel.logout.revoke.offline.tokens" = "false"
  "backchannel.logout.session.required"      = "true"
}

[mrparkers]

Hey @olivierboudet, thanks for the PR. I think supporting extra_config is fine for this resource.

As far as the backchannel attributes goes, I went ahead and opened #583 to support these as top level attributes. So you can remove the changes that adds these attributes through extra_config.

[mrparkers]

Thanks for the PR, LGTM 🚀