@Nullable
    private ClientAuthentication clientAuthentication() {
        if (clientAuthenticationSupplier != null) {
            final ClientAuthentication clientAuthentication = clientAuthenticationSupplier.get();
            return requireNonNull(clientAuthentication, "clientAuthenticationSupplier.get() returned null");
        }
        return null;
    }
Implements Token Revocation request/response flow,
as per <a href="https://datatracker.ietf.org/doc/html/rfc7009#section-2">[RFC7009], Section 2</a>.
oauth2/src/main/java/com/linecorp/armeria/common/auth/oauth2/TokenRevocation.java