§ 516. Intervention by commissioner. In any action or proceeding affecting any urban renewal program, the commissioner shall be given prompt notice thereof, and he shall take such steps in such action or proceeding as may be necessary or desirable to protect the public interest. If, in the opinion of the commissioner, it is necessary or desirable in the public interest that he intervene in any such action or proceeding he shall be permitted to do so as a matter of right. Whenever in connection with an urban renewal program, under any instrument or law, a notice in writing is required to be served upon the municipality before the institution of any action or proceeding, a copy of such notice shall be served upon the commissioner at least five days before commencement of the action or proceeding.