Move S3BinaryCacheStore from Hydra

This allows running arbitrary Nix commands against an S3 binary cache.

To do: make this a compile time option to prevent a dependency on
aws-sdk-cpp.

src/libstore/s3-binary-cache-store.hh

@@ -0,0 +1,34 @@
+#pragma once
+
+#include "binary-cache-store.hh"
+
+#include <atomic>
+
+namespace nix {
+
+class S3BinaryCacheStore : public BinaryCacheStore
+{
+protected:
+
+    S3BinaryCacheStore(std::shared_ptr<Store> localStore,
+        const Path & secretKeyFile)
+        : BinaryCacheStore(localStore, secretKeyFile)
+    { }
+
+public:
+
+    struct Stats
+    {
+        std::atomic<uint64_t> put{0};
+        std::atomic<uint64_t> putBytes{0};
+        std::atomic<uint64_t> putTimeMs{0};
+        std::atomic<uint64_t> get{0};
+        std::atomic<uint64_t> getBytes{0};
+        std::atomic<uint64_t> getTimeMs{0};
+        std::atomic<uint64_t> head{0};
+    };
+
+    const Stats & getS3Stats();
+};
+
+}