[TieredStorage] TieredStorageReader:: and HotStorageReader:: accounts() (#35031)

#### Problem
HotStorageReader and TieredStorageReader haven't implemented
accounts() that is required by AcocuntsFile.

#### Summary of Changes
This PR implements accounts() for both HotStorageReader
and TieredStorageReader

#### Test Plan
Extend the existing test to cover accounts().

accounts-db/src/tiered_storage/hot.rs

@@ -458,6 +458,24 @@ impl HotStorageReader {
             IndexOffset(index_offset.0.saturating_add(1)),
         )))
     }
+
+    /// Return a vector of account metadata for each account, starting from
+    /// `index_offset`
+    pub fn accounts(
+        &self,
+        mut index_offset: IndexOffset,
+    ) -> TieredStorageResult<Vec<StoredAccountMeta>> {
+        let mut accounts = Vec::with_capacity(
+            self.footer
+                .account_entry_count
+                .saturating_sub(index_offset.0) as usize,
+        );
+        while let Some((account, next)) = self.get_account(index_offset)? {
+            accounts.push(account);
+            index_offset = next;
+        }
+        Ok(accounts)
+    }
 }
 
 fn write_optional_fields(
@@ -1402,5 +1420,21 @@ pub mod tests {
                 storable_accounts.get(stored_info.offset);
             verify_account(&stored_meta, account, address, account_hash);
         }
+
+        // verify get_accounts
+        let accounts = hot_storage.accounts(IndexOffset(0)).unwrap();
+
+        // first, we verify everything
+        for (i, stored_meta) in accounts.iter().enumerate() {
+            let (account, address, account_hash, _write_version) = storable_accounts.get(i);
+            verify_account(stored_meta, account, address, account_hash);
+        }
+
+        // second, we verify various initial position
+        let total_stored_accounts = accounts.len();
+        for i in 0..total_stored_accounts {
+            let partial_accounts = hot_storage.accounts(IndexOffset(i as u32)).unwrap();
+            assert_eq!(&partial_accounts, &accounts[i..]);
+        }
     }
 }

accounts-db/src/tiered_storage/readable.rs

@@ -148,4 +148,15 @@ impl TieredStorageReader {
             }
         }
     }
+
+    /// Return a vector of account metadata for each account, starting from
+    /// `index_offset`
+    pub fn accounts(
+        &self,
+        index_offset: IndexOffset,
+    ) -> TieredStorageResult<Vec<StoredAccountMeta>> {
+        match self {
+            Self::Hot(hot) => hot.accounts(index_offset),
+        }
+    }
 }