The A’s signed 17-year-old Dominican shortstop Yairo Munoz on Tuesday for a $280,000 bonus, according to the Dominican Prospect League website. Munoz’s defensive skills are said to be his strongpoint, along with above-average speed. It’s tough to project how any of these international prospects will develop when they’re signed at such a young age. Will the A’s ever get a return on the $4.2 million they invested in Dominican right-hander Michael Ynoa, 20, who is coming back from Tommy John elbow surgery? The A’s are pouring lots of money into their international scouting and player development, thinking they can get a jump on teams that are focusing their big spending at the major league level …