Add GetChannelMember method to plugin API (#7930)

app/plugin_api.go

@@ -115,6 +115,10 @@ func (api *PluginAPI) UpdateChannel(channel *model.Channel) (*model.Channel, *mo
 	return api.app.UpdateChannel(channel)
 }
 
+func (api *PluginAPI) GetChannelMember(channelId, userId string) (*model.ChannelMember, *model.AppError) {
+	return api.app.GetChannelMember(channelId, userId)
+}
+
 func (api *PluginAPI) CreatePost(post *model.Post) (*model.Post, *model.AppError) {
 	return api.app.CreatePostMissingChannel(post, true)
 }