Developers proposing a Suffolk Downs casino offered the first public glimpse of their “urban oasis’’ Tuesday, outlining broad concepts behind a reinvention of the Depression-era racetrack into a luxury gambling resort under the Caesars brand. Plans would retain thoroughbred racing at the site, which straddles East Boston and Revere, remodel the existing grandstands, add a 300-room hotel and up to 10 restaurants, entertainment venues, retail shops and a spa, and 200,000 square-feet of slot machines and table games.