Multi tenant configuration: realmAuthServerUrlResolver

[sbabych]

Hello,

official keycloak-connect library has an issue related to the FrontendURL and the issuer validation.
Allow multiple auth-server-urls
Supporting different ISS when validating token

What do you think about idea to extend current Multi tenant configuration with new resolver: realmAuthServerUrlResolver
This resolver will change keycloak.grantManager.realmUrl and will help to bypass keycloak-connect library issues

Right now to achieve this I need to implement my own KeycloakMultiTenantService and override method:
get(realm: string): Promise<KeycloakConnect.Keycloak>;

Thank you!

[ferrerojosh]

You can make a PR for it and I can make a review, since you already extending and implementing your own.