Event transformation Pipeline

We need to have way to add migrators/adapters the system.

**Currently**, we have a hack function that users can call to append an item to the overall `common_transformers` list.

**Instead**, we should have this defined at the top level of the module specifying which versions/sources/event types it applies to. When loading the module, LO can extract this and add it to the pipeline then.

Search the codebase for the `common_transformers` list, the current hack function, and the module-loading path. Read how transformers are currently appended and how versions, sources, and event types are represented. Done means modules can declare applicable migrators or adapters at top level and the loader adds them to the event pipeline.

Written by the indexing model from the issue text.