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
private static BytesReference convertRoleDescriptorsMapToBytes(Map<String, Object> roleDescriptorsMap) {
        try (XContentBuilder builder = XContentBuilder.builder(XContentType.JSON.xContent())) {
            builder.map(roleDescriptorsMap);
            return BytesReference.bytes(builder);
        } catch (IOException e) {
            throw new UncheckedIOException(e);
        }
    }
    static boolean equivalentRealms(String name1, String type1, String name2, String type2) {
        if (false == type1.equals(type2)) {
            return false;
        }
        if (isFileOrNativeRealm(type1)) {
            
            return true;
        } else {
            
            return name1.equals(name2);
        }
    }
    
    public enum AuthenticationType {
        REALM,
        API_KEY,
        TOKEN,
        ANONYMOUS,
        INTERNAL
    }
    
    static class RunAsUser extends User {