Source defaults from the model instead of implicitly (#2985)

## Motivation and Context
We weren't correctly computing defaults which lead to incorrect behavior
when coupled with nullability.

## Description
Minimal changeset to source defaults from the model. Other changes:
- Unify enum parsing across client and server to always use `from_str`
in protocol tests
- Extract `PrimitiveInstantiator` from `Instantiator` so it can be used
to instantiate defaults

## Testing
- regular codegen tests

## 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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