fix(datastore): Allow saving nested byte slice

[bhshkh]

Issue: Saving nested byte slice through Datastore Go client libary throws error flattening nested structs leads to a slice of slices
Cause: Datastore does not allow saving entities with nested lists and throws error: rpc error: code = InvalidArgument desc = list_value cannot contain a Value containing another list_value.

To avoid this, the Datastore client library has a validation to prevent slice of slices. But it does not check the datatype of elements in the slice. This is incorrect since Datastore service allows saving nested list of bytes (In Go, byte is an alias for Uint8 ) . So, the library should allow such lists as well.

Fix: Correct the validation condition