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/Subject.java
assert ServiceAccountSettings.REALM_NAME.equals(realm.getName()) : "service account realm name mismatch";
            this.type = Type.SERVICE_ACCOUNT;
        } else {
            this.type = Type.USER;
        }
        this.metadata = metadata;
    }
    public User getUser() {
        return user;
    }
    public Authentication.RealmRef getRealm() {
        return realm;
    }
    public Type getType() {
        return type;
    }
    public Map<String, Object> getMetadata() {
        return metadata;
    }
    public RoleReferenceIntersection getRoleReferenceIntersection(@Nullable AnonymousUser anonymousUser) {
        switch (type) {
            case USER:
                return buildRoleReferencesForUser(anonymousUser);
            case API_KEY:
                return buildRoleReferencesForApiKey();
            case SERVICE_ACCOUNT:
                return new RoleReferenceIntersection(new RoleReference.ServiceAccountRoleReference(user.principal()));
            default:
                assert false : "unknown subject type: [" + type + "]";
                throw new IllegalStateException("unknown subject type: [" + type + "]");