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
this.metadata = metadata;
        if (user instanceof RunAsUser runAsUser) {
            authenticatingSubject = new Subject(runAsUser.authenticatingUser, authenticatedBy, version, metadata);
            
            
            
            
            effectiveSubject = new Subject(user, lookedUpBy, version, Map.of());
        } else {
            authenticatingSubject = effectiveSubject = new Subject(user, authenticatedBy, version, metadata);
        }
        this.assertApiKeyMetadata();
        this.assertDomainAssignment();
    }
    public Authentication(StreamInput in) throws IOException {
        this.user = AuthenticationSerializationHelper.readUserFrom(in);
        this.authenticatedBy = new RealmRef(in);
        if (in.readBoolean()) {
            this.lookedUpBy = new RealmRef(in);
        } else {
            this.lookedUpBy = null;
        }
        this.version = in.getVersion();
        type = AuthenticationType.values()[in.readVInt()];
        metadata = in.readMap();
        if (user instanceof RunAsUser runAsUser) {
            authenticatingSubject = new Subject(runAsUser.authenticatingUser, authenticatedBy, version, metadata);