Must Clause 4 of the framework agreement on fixed-term work concluded by ETUC, UNICE and CEEP approved by Council Directive 1999/70/EC of 28 June 1999 be interpreted as precluding a regional legislative provision, such as that at issue in the main proceedings, which expressly excludes the award and payment to staff employed by the Public Administration of Navarre who are classified as ‘employed under an administrative contract’ (a fixed-term contract) of particular additional remuneration, on the grounds that the additional remuneration in question constitutes remuneration for promotion and development in a professional career that is open only to staff classified as ‘established public officials’ (with a contract of indefinite duration)?