Geo: Replicate AlertManagement::MetricImage and IssuableMetricImage model

@dbalexandre !79083 (comment 836410166):

@seanarnold It seems this MR adds a new data type that Geo needs to sync across Geo sites. Do you mind creating a follow-up issue to tackle this? For more information, see https://docs.gitlab.com/ee/development/geo/framework.html.