Allow SDK to be built with unknown traits in models (#2547)

## Motivation and Context
Addresses #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](smithy-lang/smithy#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.

----

_By submitting this pull request, I confirm that you can use, modify,
copy, and redistribute this contribution, under the terms of your
choice._

---------

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

Author: ysaito1001

aws/sdk/build.gradle.kts

@@ -20,6 +20,7 @@ plugins {
 
 configure<software.amazon.smithy.gradle.SmithyExtension> {
     smithyBuildConfigs = files(buildDir.resolve("smithy-build.json"))
+    allowUnknownTraits = true
 }
 
 val smithyVersion: String by project

codegen-server/src/main/kotlin/software/amazon/smithy/rust/codegen/server/smithy/generators/protocol/ServerProtocolTestGenerator.kt

@@ -828,6 +828,8 @@ class ServerProtocolTestGenerator(
             FailingTest(RestJsonValidation, "RestJsonMalformedEnumMapValue_case1", TestType.MalformedRequest),
             FailingTest(RestJsonValidation, "RestJsonMalformedEnumString_case0", TestType.MalformedRequest),
             FailingTest(RestJsonValidation, "RestJsonMalformedEnumString_case1", TestType.MalformedRequest),
+            FailingTest(RestJsonValidation, "RestJsonMalformedEnumTraitString_case0", TestType.MalformedRequest),
+            FailingTest(RestJsonValidation, "RestJsonMalformedEnumTraitString_case1", TestType.MalformedRequest),
             FailingTest(RestJsonValidation, "RestJsonMalformedEnumUnion_case0", TestType.MalformedRequest),
             FailingTest(RestJsonValidation, "RestJsonMalformedEnumUnion_case1", TestType.MalformedRequest),
         )

gradle.properties

@@ -18,7 +18,7 @@ kotlin.code.style=official
 
 # codegen
 smithyGradlePluginVersion=0.6.0
-smithyVersion=1.28.1
+smithyVersion=1.29.0
 
 # kotlin
 kotlinVersion=1.7.21