When using OIDC Identity Brokering, I've found that the keycloak's built-in OIDC
Identity Brokering provider does not have the feature of forwarding some parameters
originated from a client application to an external IdP.
Such the feature might be beneficial when you host its own IdP and want to configure its
UX based on forwarded client parameters and so on.
Previously, I had realized this feature by implementing custom Authentication provider and
User Storage provider.
However, it had been too much complicated and I had the advice to use Identity Brokering.
Therefore, I've implemented this feature(Forwarding parameters) based on this OIDC
Identity Brokering built-in provider.
Forwarded parameters are prefixed with "fwd_".
It needs some documentation so that I'd like to do that if this proposal is accepted.
Also, I've created corresponding JIRA ticket.
Hope this PR is reviewed and merged.