libstore/s3: sign payloads if the request demands it

This fixes problems with S3 implementations that have mandatory payload
signing, e.g. Garage v2, AWS S3 itself, etc.

The problem manifested itself when the AWS SDK threw some error codes 99
(NETWORK_CONNECTION) with no messages and retried until it finally came
to a moment where the state machinery decided to send a
`Transfer-Encoding` header in a `Content-Encoding`/`Content-Length`-set
request with signed headers (even though payload signing is disabled),
causing the server to reject the transfer and crash the copy.

I did not debug super far what went wrong in AWS SDK, but I can confirm
this change makes transfers possible to finish with Garage v2.

Change-Id: Icc9e6a9f2afb0d760cf2d1e27816decd385a1d85
Signed-off-by: Raito Bezarius <raito@lix.systems>

lix/libstore/s3-binary-cache-store.cc

@@ -134,17 +134,22 @@ S3Helper::S3Helper(
     const std::string & profile,
     const std::string & region,
     const std::string & scheme,
-    const std::string & endpoint)
+    const std::string & endpoint
+)
     : config(makeConfig(region, scheme, endpoint))
     , client(make_ref<Aws::S3::S3Client>(
-            profile == ""
-            ? std::dynamic_pointer_cast<Aws::Auth::AWSCredentialsProvider>(
-                std::make_shared<Aws::Auth::DefaultAWSCredentialsProviderChain>())
-            : std::dynamic_pointer_cast<Aws::Auth::AWSCredentialsProvider>(
-                std::make_shared<Aws::Auth::ProfileConfigFileAWSCredentialsProvider>(profile.c_str())),
-            *config,
-            Aws::Client::AWSAuthV4Signer::PayloadSigningPolicy::Never,
-            endpoint.empty()))
+          profile == "" ? std::dynamic_pointer_cast<Aws::Auth::AWSCredentialsProvider>(
+                              std::make_shared<Aws::Auth::DefaultAWSCredentialsProviderChain>()
+                          )
+                        : std::dynamic_pointer_cast<Aws::Auth::AWSCredentialsProvider>(
+                              std::make_shared<Aws::Auth::ProfileConfigFileAWSCredentialsProvider>(
+                                  profile.c_str()
+                              )
+                          ),
+          *config,
+          Aws::Client::AWSAuthV4Signer::PayloadSigningPolicy::RequestDependent,
+          endpoint.empty()
+      ))
 {
 }