Add ability to modify Tags for an instance

[DocSavage]

Tags can be provided when creating an instance and are useful not only for specifying datastores (e.g., meshes should go to store X) but classes of a datatype (e.g., these are "user-supplied" annotations). We should allow users to add Tags after an instance is created, much like Syncs can also be added. Tags and Syncs will be the only mutable properties for a data instance.