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
public enum Type {
        USER,
        API_KEY,
        SERVICE_ACCOUNT,
    }
    private final Version version;
    private final User user;
    private final Authentication.RealmRef realm;
    private final Type type;
    private final Map<String, Object> metadata;
    public Subject(User user, Authentication.RealmRef realm) {
        this(user, realm, Version.CURRENT, Map.of());
    }
    public Subject(User user, Authentication.RealmRef realm, Version version, Map<String, Object> metadata) {
        this.version = version;
        this.user = user;
        this.realm = realm;
        
        
        if (realm == null) {
            this.type = Type.USER;
        } else if (AuthenticationField.API_KEY_REALM_TYPE.equals(realm.getType())) {
            assert AuthenticationField.API_KEY_REALM_NAME.equals(realm.getName()) : "api key realm name mismatch";
            this.type = Type.API_KEY;
        } else if (ServiceAccountSettings.REALM_TYPE.equals(realm.getType())) {