Add missing setter to PropertySerializationName property (#4617)

Adds a missing setter to the property which was overlooked as part of
https://github.com/microsoft/typespec/pull/4591.

packages/http-client-csharp/generator/Microsoft.Generator.CSharp.Customization/src/CodeGenSerializationAttribute.cs

@@ -17,7 +17,7 @@ namespace Microsoft.Generator.CSharp.Customization
         /// <summary>
         /// Gets or sets the serialization name of the property.
         /// </summary>
-        public string? PropertySerializationName { get; }
+        public string? PropertySerializationName { get; set; }
         /// <summary>
         /// Gets or sets the method name to use when serializing the property value (property name excluded)
         /// The signature of the serialization hook method must be or compatible with when invoking: