add s3 file system

[KefanXIAO]

In order to use the S3, add a S3 filesystem similar to the GCS one. The S3 client and the APIs like GetObject are also applied similar to the GCS one. Tested with my S3 bucket and it works with both integrity and good efficiency.

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[KefanXIAO]

Just signed the CLA. PTAL

[KefanXIAO]

Hi @charlesbeattie and @Augustin-Zidek , would you mind taking a look at this PR? I'm adding a functionality for S3.

[lucasb-eyer]

I recommend that you use absl::call_once instead of the static to make it thread-safe:

--- a/src/file/file_systems/s3/s3_file_system.cc
+++ b/src/file/file_systems/s3/s3_file_system.cc
@@ -22,6 +22,7 @@
 #include <vector>
 
 #include "absl/algorithm/container.h"
+#include "absl/base/call_once.h"
 #include "absl/base/no_destructor.h"
 #include "absl/base/nullability.h"
 #include "absl/functional/function_ref.h"
@@ -217,12 +218,11 @@ std::shared_ptr<Aws::S3::S3Client> S3FileSystem::Client() const {
   ClientFactory client_factory = client_factory_;
   if (client_factory == nullptr) {
     client_factory = [] {
-      static Aws::SDKOptions options;
-      static bool initialized = false;
-      if (!initialized) {
+      static absl::once_flag aws_init_once;
+      absl::call_once(aws_init_once, []() {
+        static Aws::SDKOptions options;
         Aws::InitAPI(options);
-        initialized = true;
-      }
+      });
       return std::make_shared<Aws::S3::S3Client>();
     };
   }