[keycloak-user] Register new ProviderFactorys to Mappers

We have developed a customised OIDCIdentityProviderFactory and OIDCIdentityProvider because some IdP does not fully adhere to the standards. However, when using this Provider, the UserAttributeMapper (and other mappers) are not used because they have a fixed list of COMPATIBLE_PROVIDERS. We would suggest adding a registerCompatibleProvider to the IdentityProviderMapper in order to extend the usage of these mappers. Just checking before we take the effort of creating a nice PR: is this something you would see fly? If yes, would you suggest changing COMPATIBLE_PROVIDERS to a List<String> or a List<IdentityProviderFactory> and subsequently changing the getCompatibleProviders() implementations? Thanks, Chris