It is hereby determined that the purposes of this chapter are public purposes and that the municipalities will be performing an essential governmental function in the exercise of the powers conferred upon them by this chapter. The state covenants with the purchasers and all subsequent holders and transferees of notes and bonds issued by the municipality, in consideration of the acceptance of and payment for the notes and bonds, that the principal and interest of notes and bonds of the municipality issued pursuant to this chapter shall at all times be free from taxation, except for estate and gift taxes, imposed by the state or by any political subdivision thereof. Municipalities are authorized to include this covenant of the state in any agreement with the holder of such notes or bonds.