Spawning from https://github.com/element-hq/synapse/pull/17187#discussion_r1619492779 around wanting to put `SlidingSyncBody` (parse the request in the rest layer), `SlidingSyncConfig` (from the rest layer, pass to the handler), `SlidingSyncResponse` (pass the response from the handler back to the rest layer to respond) somewhere that doesn't contaminate the imports and cause circular import issues.
- Moved Pydantic parsing models to `synapse/types/rest`
- Moved handler types to `synapse/types/handlers`
