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
getUser().metadata(),
                getUser().enabled()
            );
        }
        return new Authentication(user, getAuthenticatedBy(), getLookedUpBy(), getVersion(), getAuthenticationType(), getMetadata());
    }
    /**
     * Returns {@code true} if the effective user belongs to a realm under a domain.
     * See also {@link #getDomain()} and {@link #getSourceRealm()}.
     */
    public boolean isAssignedToDomain() {
        return getDomain() != null;
    }
    /**
     * Returns the {@link RealmDomain} that the effective user belongs to.
     * A user belongs to a realm which in turn belongs to a domain.
     *
     * The same username can be authenticated by different realms (e.g. with different credential types),
     * but resources created across realms cannot be accessed unless the realms are also part of the same domain.
     */
    public @Nullable RealmDomain getDomain() {
        return getSourceRealm().getDomain();
    }
    public boolean isAuthenticatedWithServiceAccount() {
        return ServiceAccountSettings.REALM_TYPE.equals(getAuthenticatedBy().getType());
    }
    /**
     * Whether the authenticating user is an API key, including a simple API key or a token created by an API key.
     */
    public boolean isAuthenticatedAsApiKey() {