Introduce concept of Coder, extract ActiveRecordCoder (#48)

FixtureKit currently only supports saving and loading fixture data from SQL (via ActiveRecord). As we look to support other data sources — for example, Redis or other non-SQL stores — we need an abstraction that allows different persistence strategies to plug in alongside the existing SQL behavior.

This PR introduces that abstraction with a `Coder` base class and extracts the existing SQL logic into `ActiveRecordCoder` as the first step.