A district may provide for the pension of officers or employees and the creation of a special fund for the purpose of paying such pension and the accumulation of contributions to said fund from the revenues of the district, the wages of officers or employees, voluntary contributions, gifts, donations, or any source of revenue, not inconsistent with the general powers of the board, and may contract with any insurance corporation or any other insurance carrier for the establishment and maintenance of group insurance, a system of group annuities, and a pension service for such officers and employees, and may provide for the terms and conditions under which such pensions, group insurance, or group annuities shall be allowed, and for the time and extent of service of officers or employees before such pensions, group insurance, or group annuities shall be available to them.