CMA CGM, the world's third-biggest container ship company, will begin service to the Conley Terminal in South Boston every other Thursday starting Dec. 7, the Massachusetts Port Authority said. The move will increase total container-shipping business to Boston's port by 4 to 5 percent annually and will also offer the first direct-shipping service between Boston and two European ports, Rotterdam and Liverpool. Boston's port now gets Mediterranean Shipping Co. vessels twice weekly from Europe and China Ocean Shipping Co. vessels weekly from China, along with a weekly Boston-New York Columbia Coastal Transport barge that moves containers to and from ships in New York Harbor.