Evaluating the impact, implementation experience and political economy of primary care networks in Kenya: protocol for a mixed methods study by Amboko, B, Nzinga, J, Tsofa, B, Mugo, P, Musiega, A, Maritim, B, Wong, E, Mazzilli, C, Ng’ang’a, W, Hagedorn, B, Turner, G, Musuva, A, Murira, F, Ravishankar, N, Hussein, S, Barasa, E