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
import org.elasticsearch.xpack.core.security.authc.service.ServiceAccountSettings;
import org.elasticsearch.xpack.core.security.authc.support.AuthenticationContextSerializer;
import org.elasticsearch.xpack.core.security.user.AnonymousUser;
import org.elasticsearch.xpack.core.security.user.AsyncSearchUser;
import org.elasticsearch.xpack.core.security.user.SecurityProfileUser;
import org.elasticsearch.xpack.core.security.user.SystemUser;
import org.elasticsearch.xpack.core.security.user.User;
import org.elasticsearch.xpack.core.security.user.XPackSecurityUser;
import org.elasticsearch.xpack.core.security.user.XPackUser;
import java.io.IOException;
import java.io.UncheckedIOException;
import java.util.Base64;
import java.util.Collections;
import java.util.EnumSet;
import java.util.HashMap;
import java.util.List;
import java.util.Locale;
import java.util.Map;
import java.util.Objects;
import static org.elasticsearch.xcontent.ConstructingObjectParser.constructorArg;
import static org.elasticsearch.xcontent.ConstructingObjectParser.optionalConstructorArg;
import static org.elasticsearch.xpack.core.security.authc.Authentication.RealmRef.newAnonymousRealmRef;
import static org.elasticsearch.xpack.core.security.authc.Authentication.RealmRef.newApiKeyRealmRef;
import static org.elasticsearch.xpack.core.security.authc.Authentication.RealmRef.newInternalAttachRealmRef;
import static org.elasticsearch.xpack.core.security.authc.Authentication.RealmRef.newInternalFallbackRealmRef;
import static org.elasticsearch.xpack.core.security.authc.Authentication.RealmRef.newServiceAccountRealmRef;
import static org.elasticsearch.xpack.core.security.authc.AuthenticationField.ANONYMOUS_REALM_NAME;
import static org.elasticsearch.xpack.core.security.authc.AuthenticationField.ANONYMOUS_REALM_TYPE;
import static org.elasticsearch.xpack.core.security.authc.AuthenticationField.API_KEY_REALM_NAME;
import static org.elasticsearch.xpack.core.security.authc.AuthenticationField.API_KEY_REALM_TYPE;