[SYS-7856] Allow empty prefix in RocksDB-Cloud

Summary:
See https://github.com/rockset/rs/pull/24132#discussion_r1401179422

Test Plan:
Internal test

Reviewers:

cloud/cloud_file_system.cc

@@ -125,18 +125,27 @@ BucketOptions::BucketOptions() {
       "ROCKSDB_CLOUD_TEST_REGION", "ROCKSDB_CLOUD_REGION", &region_);
 }
 
+void BucketOptions::SetName() {
+  if (bucket_.empty()) {
+    name_.clear();
+  } else {
+    name_ = prefix_ + bucket_;
+  }
+}
+
 void BucketOptions::SetBucketName(const std::string& bucket,
                                   const std::string& prefix) {
   if (!prefix.empty()) {
     prefix_ = prefix;
   }
 
   bucket_ = bucket;
-  if (bucket_.empty()) {
-    name_.clear();
-  } else {
-    name_ = prefix_ + bucket_;
-  }
+  SetName();
+}
+
+void BucketOptions::SetPrefix(std::string prefix) {
+  prefix_ = std::move(prefix);
+  SetName();
 }
 
 // Initializes the bucket properties

include/rocksdb/cloud/cloud_file_system.h

@@ -129,12 +129,16 @@ class BucketOptions {
   std::string object_;  // The object path for the bucket
   std::string region_;  // The region for the bucket
   std::string name_;    // The name of the bucket (prefix_ + bucket_)
+
+  void SetName();
+
  public:
   BucketOptions();
   // Sets the name of the bucket to be the new bucket name.
   // If prefix is specified, the new bucket name will be [prefix][bucket]
   // If no prefix is specified, the bucket name will use the existing prefix
   void SetBucketName(const std::string& bucket, const std::string& prefix = "");
+  void SetBucketPrefix(std::string prefix);
   const std::string& GetBucketPrefix() const { return prefix_; }
   const std::string& GetBucketName(bool full = true) const {
     if (full) {