text: Title: Add `secret` attribute to columns
  Body: We have a `Secret` column type:
  
  https://piccolo-orm.readthedocs.io/en/latest/piccolo/schema/column_types.html#secret
  
  The reason this exists is to prevent inadvertent leakage of sensitive data. You can use it like this:
  
  ```python
  class Property(Table):
      address = Text()
      code = Secret()
  
  >>> await Property.select(exclude_secrets=True)
  [{'address': 'Buckingham Palace'}]
  ```
  
  `PiccoloCRUD` also supports it.
  
  The issue is `Secret` is a subclass of `Varchar` - there's no way to have a secret `Integer` for example.
  
  The change will involve adding a `secret` parameter to `Column` (and hence is accessible to all subclasses). The `Secret` column type will still be kept for backwards compatibility, but will be deprecated.
  
  ```python
  class Property(Table):
      address = Text()
      code = Varchar(secret=True)
      value = Integer(secret=True)
  ```
  