Enhancing authenticity, diagnosticity and equivalence (AD-Equiv) in multicentre OSCE exams in health professionals education: protocol for a complex intervention study by Yeates, P, Maluf, A, Kinston, R, Cope, N, McCray, G, Cullen, K, O'Neill, V, Cole, A, Goodfellow, R, Vallender, R, Chung, C-W, McKinley, RK, Fuller, R, Wong, G