Add injection hook for structures in no service (#3001)

## Motivation and Context
This change will allow to render structures in no service closures with
a decorator.

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [ ] I have updated `CHANGELOG.next.toml` if I made changes to the AWS
SDK, generated SDK code, or SDK runtime crates

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choice._

Signed-off-by: Daniele Ahmed <ahmeddan@amazon.de>