add support for nullable struct members when generating AWS SDKs (#2916)

## Motivation and Context
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->
smithy-rs#1767 aws-sdk-rust#536

## Description
<!--- Describe your changes in detail -->
This PR adds support for nullability i.e. much less unwraps will be
required when using the AWS SDK. For generic clients, this new behavior
can be enabled in codegen by setting `nullabilityCheckMode: "Client"` in
their codegen config:
```
      "plugins": {
        "rust-client-codegen": {
          "codegen": {
            "includeFluentClient": false,
            "nullabilityCheckMode": "CLIENT_CAREFUL"
          },
     }
```


## Testing
<!--- Please describe in detail how you tested your changes -->
<!--- Include details of your testing environment, and the tests you ran
to -->
<!--- see how your change affects other areas of the code, etc. -->
Ran existing tests

## Checklist
<!--- If a checkbox below is not applicable, then please DELETE it
rather than leaving it unchecked -->
- [x] I have updated `CHANGELOG.next.toml` if I made changes to the
smithy-rs codegen or runtime crates
- [x] 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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Co-authored-by: John DiSanti <jdisanti@amazon.com>
Co-authored-by: Russell Cohen <rcoh@amazon.com>