FORMER Assistant Secretary of State Elliott Abrams has pleaded guilty to two misdemeanor charges of withholding information from Congress about a supply network to finance the Nicaraguan contras.Mr. Abrams became the seventh person to plead guilty in the Iran-contra scandal and faces a minimum of 30 days in jail on each count. He could get up to two years. Fines could total $100,000 on each count, with a minimum penalty of $100. Craig Gillen, a lawyer representing the independent counsel's office in the case, said Abrams has agreed to cooperate with authorities probing the Reagan administration scandal. Mr. Gillen is expected to seek a reduced sentence. "This is clearly an important development that should enable us to move more quickly and with thoroughness to the conclusion of our investigation," independent counsel Lawrence Walsh said in a statement. The independent prosecutor charged that Abrams withheld information from Congress in October 1986 by denying knowledge of private individuals who had been approached to provide funds to the Nicaraguan contras. In the second count, Abrams was charged with deceiving Congress when he denied knowing of any foreign governments that were helping supply the contras.