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>

src/ReC.Infrastructure/Options/Shared/EntityBaseOptions.cs

@@ -2,11 +2,11 @@
 
 public record EntityBaseOptions()
 {
-    public Dictionary<string, string> Columns { get; init; } = [];
+    public Dictionary<string, string> ColumnName { get; init; } = [];
 
     public void EnsureProperties(IEnumerable<string> propertyNames)
     {
-        var missingProperties = propertyNames.Except(Columns.Select(col => col.Key)).ToList();
+        var missingProperties = propertyNames.Except(ColumnName.Select(col => col.Key)).ToList();
 
         if (missingProperties.Count != 0)
             throw new InvalidOperationException($"The following properties are not configured: {string.Join(", ", missingProperties)}");