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
builder.field(User.Fields.REALM_TYPE.getPreferredName(), getAuthenticatedBy().getType());
            if (getAuthenticatedBy().getDomain() != null) {
                builder.field(User.Fields.REALM_DOMAIN.getPreferredName(), getAuthenticatedBy().getDomain().name());
            }
        }
        builder.endObject();
        builder.field(User.Fields.AUTHENTICATION_TYPE.getPreferredName(), getAuthenticationType().name().toLowerCase(Locale.ROOT));
        if (isApiKey()) {
            this.assertApiKeyMetadata();
            final String apiKeyId = (String) this.metadata.get(AuthenticationField.API_KEY_ID_KEY);
            final String apiKeyName = (String) this.metadata.get(AuthenticationField.API_KEY_NAME_KEY);
            if (apiKeyName == null) {
                builder.field("api_key", Map.of("id", apiKeyId));
            } else {
                builder.field("api_key", Map.of("id", apiKeyId, "name", apiKeyName));
            }
        }
    }
    private void assertApiKeyMetadata() {
        assert (false == isAuthenticatedAsApiKey()) || (this.metadata.get(AuthenticationField.API_KEY_ID_KEY) != null)
            : "API KEY authentication requires metadata to contain API KEY id, and the value must be non-null.";
    }
    private void assertDomainAssignment() {
        if (Assertions.ENABLED) {
            if (isAssignedToDomain()) {
                assert false == isApiKey();
                assert false == isServiceAccount();
                assert false == isAuthenticatedAnonymously();
                assert false == isAuthenticatedInternally();
            }