No way to SetBackgroundThreads ( High and Low) by options file (ini format)

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### Expected behavior
i want to use DBOptions section file, which like in https://github.com/facebook/rocksdb/blob/master/examples/rocksdb_option_file_example.ini

to set high and low background threads

### Actual behavior

options.env->SetBackgroundThreads(4, rocksdb::Env::LOW);

options.env->SetBackgroundThreads(8, rocksdb::Env::HIGH);

### Steps to reproduce the behavior