Support buffers for DataStore operations

[Sleitnick]

Luau buffers can be stored in Roblox DataStores. This data is tagged specially. It would be nice if rbxcloud could interact with this data appropriately.

e.g. a buffer stored in a DataStore may have a format similar to:

[{
   "m": null,
   "t": "buffer",
   "zbase64": "..."
}]