The Senate, by “unanimous consent” Thursday, approved legislation authorizing the sale of a special first-class postage stamp that would help finance breast cancer research. The measure, which sets up a two-year demonstration project to sell the special stamp for up to 8 cents more than the first-class rate, currently 32 cents, was sent to President Clinton. Although most of the extra proceeds would go toward government breast cancer research, it is unclear how much would be raised because such a stamp has never before been offered in the United States and the extent of administrative costs are sketchy.