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 User partialReadUserFrom(String username, StreamInput input) throws IOException {
            String[] roles = input.readStringArray();
            Map<String, Object> metadata = input.readMap();
            String fullName = input.readOptionalString();
            String email = input.readOptionalString();
            boolean enabled = input.readBoolean();
            User outerUser = new User(username, roles, fullName, email, metadata, enabled);
            boolean hasInnerUser = input.readBoolean();
            if (hasInnerUser) {
                User innerUser = readUserFrom(input);
                assert false == User.isInternal(innerUser) : "authenticating user cannot be internal";
                return new RunAsUser(outerUser, innerUser);
            } else {
                return outerUser;
            }
        }
        private static void doWriteUserTo(User user, StreamOutput output) throws IOException {
            if (user instanceof RunAsUser runAsUser) {
                User.writeUser(user, output);
                output.writeBoolean(true);
                User.writeUser(runAsUser.authenticatingUser, output);
            } else {
                
                User.writeUser(user, output);
            }
            output.writeBoolean(false); 
        }
    }
}