Support custom model dumping in Relation.save

In our documentation, we should encourage overriding model.model_dump and add support for doing this with non-Pydantic classes, so that people can do "write exactly this content" rather than "update this content" if they want to.

Start by reading the Relation.save implementation and the documentation around model.model_dump. Check how saving currently handles Pydantic and non-Pydantic models, then identify the relevant tests. Done means non-Pydantic classes can request exact-content writes and the documentation explains overriding model.model_dump.

Written by the indexing model from the issue text.