Pop into any upscale coffee shop on Los Angeles’ affluent Westside, and you’re bound to bump into a jewelry designer. Jewelry is the art du jour in many major cities, especially L.A. But even in a town packed with ­people turning out baubles, Susan Foster stands out. The designer and retailer, who opened her eponymous business in 1999 while in her 20s, not only creates one of the most reliably glamorous fine jewelry collections on the Left Coast, she also operates a beautiful shop, in the ultimate la-la-land neighborhood of Brentwood, curated carefully with her line and a handful of