JWT-Auth: Support for Keycloak-Style nested roles

The rolesClaim for a JWT Token, as documented in https://solr.apache.org/guide/8_11/jwt-authentication-plugin.html#configuration-parameters, does not support "nested roles".

That is, consider the following claim, as returned by [keycloak|https://www.keycloak.org/] if the user has the role user for the client solr:

"resource_access": {
    "solr": {
      "roles": [
        "user"
      ]
    },
    "account": {
      "roles": [
        "manage-account",
        "manage-account-links",
        "view-profile"
      ]

   }

Here a nested roles claim would have to apply to match. Something like rolesClaim="resource_access.solr.roles"

This is currently not supported. I am working on a Pull Request.