Allow SDK to be built with unknown traits in models (upgrading Smithy to...

Allow SDK to be built with unknown traits in models (upgrading Smithy to 1.29.0 as part of it) (#2547)

## Motivation and Context
Addresses https://github.com/awslabs/smithy-rs/issues/2104

## Description
This PR allows us to build `aws-sdk-rust` even with unknown traits
present in the AWS models. This requires `Smithy` 1.29.0 that includes
[support](https://github.com/awslabs/smithy/pull/1685) for an extension
property,
[allowUnknownTraits](https://smithy.io/2.0/guides/building-models/gradle-plugin.html?highlight=allowunknowntraits#smithy-extension-properties

).

## Testing
Manually added a unknown trait into an AWS model like so:
```
diff --git a/aws/sdk/aws-models/sts.json b/aws/sdk/aws-models/sts.json
--- a/aws/sdk/aws-models/sts.json
+++ b/aws/sdk/aws-models/sts.json
@@ -69,6 +69,7 @@
                 "aws.auth#sigv4": {
                     "name": "sts"
                 },
+                "aws.protocols#awsQuery2": {},
                 "aws.protocols#awsQuery": {},
```
and confirmed `/gradlew :aws:sdk:assemble` built SDK successfully. Also
verified that without `allowUnknownTraits = true` in
`aws/sdk/build.gradle.kts`, `/gradlew :aws:sdk:assemble` failed with the
above unknown trait.

----

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choice._

---------

Co-authored-by: Yuki Saito <awsaito@amazon.com>