By Ahmad Ghaddar LONDON (Reuters) -Oil prices rose on Thursday after an oil tanker was boarded by an armed group in Oman, raising the prospect of escalating conflict in the Middle East. Brent crude futures gained $1.03, or 1.3%, to $77.83 a barrel by 0916 GMT and U.S. West Texas Intermediate crude futures rose 98 cents, or 1.4%, to $72.35, though gains were capped by a surprise build in U.S. crude stockpiles. The United Kingdom Maritime Trade Operations (UKMTO) authority received a report on Thursday that a vessel about 50 nautical miles east of Oman’s coast was boarded by four to five armed i…