Add property_bag to FileOptions (#9030)

Summary:
Add a property_bag option in FileOptions for direct FileSystem users to pass custom properties to the provider in APIs such as NewRandomAccessFile, NewWritableFile etc. This field will be ignored/not populated by RocksDB.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9030

Reviewed By: zhichao-cao

Differential Revision: D31630643

Pulled By: anand1976

fbshipit-source-id: 1e1ddc5e2933ecada99a94eada5f309b674a03e8

include/rocksdb/file_system.h

@@ -25,6 +25,7 @@
 #include <memory>
 #include <sstream>
 #include <string>
+#include <unordered_map>
 #include <vector>
 
 #include "rocksdb/env.h"
@@ -90,6 +91,14 @@ struct IOOptions {
   // Type of data being read/written
   IOType type;
 
+  // EXPERIMENTAL
+  // An option map that's opaque to RocksDB. It can be used to implement a
+  // custom contract between a FileSystem user and the provider. This is only
+  // useful in cases where a RocksDB user directly uses the FileSystem or file
+  // object for their own purposes, and wants to pass extra options to APIs
+  // such as NewRandomAccessFile and NewWritableFile.
+  std::unordered_map<std::string, std::string> property_bag;
+
   IOOptions() : timeout(0), prio(IOPriority::kIOLow), type(IOType::kUnknown) {}
 };