We would like to revisit the `Authentication` class to make it provide better and easier to use interface to the consumers. 

#79809 enables run-as for all authentication schemes in addition to `realm` which is already supported. This adds more complexity to an `Authentication` object and how it should be used. For an example, building the role associated to an user now has to consider whether the user is the authenticated user or run-as user and the authentication scheme of the authenticated user. The existing `Authentication` class can answer all these question today. But its interface and internals are not aligned for these questions and the usage is error prone. For example, the `Authentication` object itself does not know whether the user has run-as. This information is kept by the `User` object. If the `User` object has an `authenticatedUser`, it's the run-as user. Otherwise it is not. But the information about realms for each `User` is directly kept in `Authentication`. Therefore, for full picture, the caller must check both `User` and `Authentication` realms and understands the nuance of their differenent combinations. 

- [x] Refactor CompositeRolesStore to separate roleDescriptors resolving (#80926)
- [ ] Move effective role descriptor checking from Resolver to CompositeRolesStore
- [x] https://github.com/elastic/elasticsearch/issues/81192
- [ ] Replace Authentication with AuthenticationContext in places other than CompositeRolesStore
x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authc/Authentication.java
public String getType() {
            return type;
        }
        /**
         * Returns the domain assignment for the realm, if one assigned, or {@code null} otherwise, as per the
         * {@code RealmSettings#DOMAIN_TO_REALM_ASSOC_SETTING} setting.
         */
        public @Nullable RealmDomain getDomain() {
            return domain;
        }
        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;
            RealmRef realmRef = (RealmRef) o;
            if (nodeName.equals(realmRef.nodeName) == false) return false;
            if (type.equals(realmRef.type) == false) return false;
            return Objects.equals(domain, realmRef.domain);
        }
        @Override
        public int hashCode() {
            int result = nodeName.hashCode();
            result = 31 * result + name.hashCode();
            result = 31 * result + type.hashCode();
            if (domain != null) {
                result = 31 * result + domain.hashCode();
            }
            return result;
        }
        @Override