GH-9608: Add GetUsersInChannel to plugin API (#9643)

* add GetUsersInChannel to plugin api

* compute offset value instead of page

* Add version comment

plugin/api.go

@@ -176,6 +176,11 @@ type API interface {
 	// DeleteChannelMember deletes a channel membership for a user.
 	DeleteChannelMember(channelId, userId string) *model.AppError
 
+	// GetUsersInChannel gets users in given channel.
+	//
+	// Minimum server version: 5.6
+	GetUsersInChannel(channelId string, page int, perPage int) ([]*model.User, *model.AppError)
+
 	// CreatePost creates a post.
 	CreatePost(post *model.Post) (*model.Post, *model.AppError)