Update timestamp defaults to CURRENT_TIMESTAMP

Changed default value for `created_at` and `updated_at` columns to `CURRENT_TIMESTAMP` for consistency and standardization. This ensures database entries have automatic timestamping based on the current time. Additionally, improved handling of default values in SQL generation to accommodate new logic.

src/expression/generator.cr

@@ -411,7 +411,11 @@ module Expression
           else
             sb << column.name
             sb << " " << @adapter.sql_type(column.type)
-            sb << " DEFAULT CURRENT_TIMESTAMP " if [:created_at, :updated_at].includes?(column.name)
+            if [:created_at, :updated_at].includes?(column.name)
+              sb << " DEFAULT " << column.default
+            elsif column.default
+              sb << " DEFAULT " << column.default
+            end
             sb << " NOT NULL" unless column.null?
             sb << " UNIQUE" if column.unique?
           end

src/table.cr

@@ -345,8 +345,8 @@ module Cql
     # timestamps
     # ```
     def timestamps
-      timestamp :created_at, default: Time.local
-      timestamp :updated_at, default: Time.local
+      timestamp :created_at, default: "CURRENT_TIMESTAMP"
+      timestamp :updated_at, default: "CURRENT_TIMESTAMP"
     end
 
     # Adds a new column to the table.