Add ability to chose column names for timestamps

[AkioSarkiz]

Right now we cannot customize column names of timestamp fields in our collection. The idea is to give us a way to customize column names of timestamp. Here is example how I see it.

Why?
https://wiki.postgresql.org/wiki/Don't_Do_This#Don.27t_use_upper_case_table_or_column_names

Example of implementation

export const Articles: CollectionConfig = {
  slug: 'articles',

  timestamps: {
      enabled: true,
      createdAt: 'created_at',
      updatedAt: 'updated_at',
   }

  fields: [
       // ...
   ]
}