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
*/
    @Deprecated
    public User getUser() {
        return user;
    }
    /**
     * Use {@code getAuthenticatingSubject().getRealm()} instead.
     */
    @Deprecated
    public RealmRef getAuthenticatedBy() {
        return authenticatedBy;
    }
    /**
     * The use case for this method is largely trying to tell whether there is a run-as user
     * and can be replaced by {@code isRunAs}
     */
    @Deprecated
    public RealmRef getLookedUpBy() {
        return lookedUpBy;
    }
    /**
     * Get the realm where the effective user comes from.
     * The effective user is the es-security-runas-user if present or the authenticated user.
     *
     * Use {@code getEffectiveSubject().getRealm()} instead.
     */
    @Deprecated
    public RealmRef getSourceRealm() {
        return lookedUpBy == null ? authenticatedBy : lookedUpBy;
    }