Transformer improvements & customization

[CarsonF]

• Allow custom transformer instance to be passed into Connection constructor options
• Updated transformer methods to be protected instead of private and have explicit any return types. This allows subclasses to use the is* methods and override the transform* methods to return whatever they want.
• Added is* and transform* for temporal types. Currently maintaining compatibility with previous functionality. subclasses can do something different if they wish. I, for instance, want to return Luxon objects.
• Updated parameters to have unknown instead of any which makes the code type safe.