Enable Keycloak kc_idp_hint query string

[BZValoche]

Summary

Add an option in, oidc.config to add a custom query string to the authentication request.

Motivation

I would like to use Keycloak's kc_id_hint query string parameter, so the users connecting to ArgoCD are automatically redirected to an external idp provider configured in Keycloak

Proposal

Add an option in oidc.config, for example extra_query_string, where one would put, in this case, kc_idp_hint=myidp.
Alternatively, use an environment variable, like KC_IDP_HINT, which, if set, would be added to the URL juste before redirecting the user in (*ClientApp).HandleLogin