GH-46214: [C++] Improve S3 client initialization

Author: pitrou

cpp/src/arrow/filesystem/s3fs.cc

@@ -1108,7 +1108,12 @@ class EndpointProviderCache {
 
 class ClientBuilder {
  public:
-  explicit ClientBuilder(S3Options options) : options_(std::move(options)) {}
+  explicit ClientBuilder(S3Options options)
+      : options_(std::move(options)),
+        // The ClientConfiguration constructor always does a current region lookup
+        // via EC2 metadata, unless IMDS is disabled (GH-46214).
+        client_config_(/*useSmartDefaults=*/true, options_.smart_defaults,
+                       /*shouldDisableIMDS=*/true) {}
 
   const Aws::Client::ClientConfiguration& config() const { return client_config_; }
 

cpp/src/arrow/filesystem/s3fs.h

@@ -96,6 +96,12 @@ class ARROW_EXPORT S3RetryStrategy {
 
 /// Options for the S3FileSystem implementation.
 struct ARROW_EXPORT S3Options {
+  /// \brief Smart defaults for option values
+  ///
+  /// The possible values for this setting are explained in the AWS docs:
+  /// https://docs.aws.amazon.com/sdkref/latest/guide/feature-smart-config-defaults.html
+  const char* smart_defaults = "standard";
+
   /// \brief AWS region to connect to.
   ///
   /// If unset, the AWS SDK will choose a default value.  The exact algorithm