Add joint_skymap field to ExternalCoincInfo record schema

Does as the title says. though notably it commits to sending a maximum of one skymap in kafka notices. This is necessary unless we ask both SCiMMA and GCN to increase the default kafka message limit of 1MB. Doing so would make both producer and consumer code more complicated, so I think it's best if we only send one skymap. I think combined skymaps should be at least as good as the better of the two individual skymaps, so this shouldn't be a problem.