Fix aws-sdk-rust#930 (PutSnapshotBlock) (#3126)

## Motivation and Context
A regression in 0.56 caused `X-Amz-Content-sha256` to be omitted when
the `UnsignedPayload` trait was present. This re-enables the old
behavior and fixes this operation.
<!--- Why is this change required? What problem does it solve? -->
<!--- If it fixes an open issue, please link to the issue here -->

## Description
Enable the `x-amz-content-sha256` header whenever the unsigned payload
trait is present.

Old code:
https://github.com/awslabs/smithy-rs/blob/smithy-rs-release-0.54.x/aws/sdk-codegen/src/main/kotlin/software/amazon/smithy/rustsdk/SigV4SigningDecorator.kt#L168

## Testing
- [x] Manual integration test
- [x] Modeled protocol test that ensures that header is set
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to -->
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## 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

----

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

CHANGELOG.next.toml

@@ -16,3 +16,9 @@ message = "Upgrade `ring` to 0.17.5."
 references = ["smithy-rs#3112", "smithy-rs#3116"]
 meta = { "breaking" = false, "tada" = false, "bug" = false, "target" = "client" }
 author = "jdisanti"
+
+[[aws-sdk-rust]]
+message = "Fix aws-sdk-rust#930 (PutSnapshotBlock)"
+references = ["smithy-rs#3126", "aws-sdk-rust#930"]
+meta = { "breaking" = false, "tada" = false, "bug" = true }
+author = "rcoh"

aws/sdk-codegen/src/main/kotlin/software/amazon/smithy/rustsdk/SigV4AuthDecorator.kt

@@ -226,7 +226,7 @@ private class AuthOperationCustomization(private val codegenContext: ClientCodeg
                 if (authSchemes.containsKey(SigV4Trait.ID)) {
                     val unsignedPayload = section.operationShape.hasTrait<UnsignedPayloadTrait>()
                     val doubleUriEncode = unsignedPayload || !disableDoubleEncode(codegenContext.serviceShape)
-                    val contentSha256Header = needsAmzSha256(codegenContext.serviceShape)
+                    val contentSha256Header = needsAmzSha256(codegenContext.serviceShape) || unsignedPayload
                     val normalizeUrlPath = !disableUriPathNormalization(codegenContext.serviceShape)
                     rustTemplate(
                         """

aws/sdk-codegen/src/test/kotlin/software/amazon/smithy/rustsdk/SigV4AuthDecoratorTest.kt

@@ -0,0 +1,64 @@
+/*
+ * Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+package software.amazon.smithy.rustsdk
+
+import org.junit.jupiter.api.Test
+import software.amazon.smithy.rust.codegen.core.testutil.asSmithyModel
+
+class SigV4AuthDecoratorTest {
+    private val modelWithSigV4AuthScheme = """
+        namespace test
+
+        use aws.auth#sigv4
+        use aws.api#service
+        use aws.protocols#restJson1
+        use smithy.rules#endpointRuleSet
+        use aws.auth#unsignedPayload
+        use smithy.test#httpRequestTests
+
+        @auth([sigv4])
+        @sigv4(name: "dontcare")
+        @restJson1
+        @endpointRuleSet({
+            "version": "1.0",
+            "rules": [{ "type": "endpoint", "conditions": [], "endpoint": { "url": "https://example.com" } }],
+            "parameters": {
+                "endpoint": { "required": true, "type": "string", "builtIn": "SDK::Endpoint" },
+            }
+        })
+        @service(sdkId: "dontcare")
+        service TestService { version: "2023-01-01", operations: [SomeOperation] }
+        structure SomeOutput { something: String }
+
+        structure SomeInput {
+            @httpPayload
+            something: Bytestream
+         }
+
+        @streaming
+        blob Bytestream
+
+        @httpRequestTests([{
+            id: "unsignedPayload",
+            protocol: restJson1,
+            method: "POST",
+            uri: "/",
+            params: {
+                something: "hello"
+            },
+            headers: {
+                "x-amz-content-sha256": "UNSIGNED-PAYLOAD",
+            },
+        }])
+        @unsignedPayload
+        @http(uri: "/", method: "POST")
+        operation SomeOperation { input: SomeInput, output: SomeOutput }
+    """.asSmithyModel()
+
+    @Test
+    fun unsignedPayloadSetsCorrectHeader() {
+        awsSdkIntegrationTest(modelWithSigV4AuthScheme) { _, _ -> }
+    }
+}