Rename Columns to ColumnName; add Attributes folder

Renamed the Columns property to ColumnName in EntityBaseOptions.cs,
updating all references accordingly. Added an "Attributes" folder
entry to ReC.Domain.csproj for future organization.

src/ReC.Domain/ReC.Domain.csproj

@@ -6,4 +6,8 @@
     <Nullable>enable</Nullable>
   </PropertyGroup>
 
+  <ItemGroup>
+    <Folder Include="Attributes\" />
+  </ItemGroup>
+
 </Project>