## What
On expanded relations, mutate type using SetRelation, an alias for SetRequired with SetNonNullable.
## Why
Simplifies implementation in client code.
## How
* Export SetRelation type util
* Update codegen template
Extra scope:
* Improve codegen error handling when processing x-expanded-relations
* Add `eager` sub-categorization to x-expanded-relations to highlight relations that are loaded using TypeORM eager.
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