datastore: support `map[string]interface{}` as a Property

**Is your feature request related to a problem? Please describe.**

I'd like to be able to save `map[string]string` fields of a structure as a nested entity.

**Describe the solution you'd like**

Will be great if `datastore.Property` supported `map[string]interface{}` as a way to save nested structs

**Describe alternatives you've considered**

For now, just use custom `Load`/`Save` implementations of `PropertyLoadSaver`