The Clinton administration unveiled yesterday a new plan to oversee the District of Columbia. The deal calls for Federal managers to take over key local government functions, including tax collection and the prison system. Many local officials welcomed the move, which will relieve the city of some major costs like pensions and Medicaid. Still, the Clinton plan avoids tackling some of the urgent issues facing the half million people of the District of Columbia — most notably the question of democracy in the Federal enclave.