default Set<String> getAccessTokens() {
        return new HashSet<>();
    }
Gets descendant OAuth access tokens.
The revocation of a refresh token may cause the revocation of related
tokens and the underlying authorization grant. If a refresh token is
revoked, the authorization server SHOULD
also invalidate all access tokens based on the same authorization
grant. Here, we track the access tokens.

@return the access tokens
@since 6.2
support/cas-server-support-oauth-api/src/main/java/org/apereo/cas/ticket/refreshtoken/OAuth20RefreshToken.java