SstFileReader in Java API

[richardyc]

C++ currently has API for reading SST files with SstFileReader sst_file_reader.h. However, rocksdbjni does not have SstFileReader API. Having this over the current option DB::IngestExternalFile() could bring support for:

• Ability to read sst files directly pulled from Snapshots. Current DB::IngestExternalFile() will raise

Exception in thread "main" org.rocksdb.RocksDBException: External file version not found

• Ability to iterate through a sst file without the overhead of creating a new db

[miasantreble]

Thanks @richardyc , will you be interested in contributing a PR for this?

[richardyc]

I should be able to contribute a PR for this in the coming weeks. However, I don't have a lot of JNI developing experience so probably need to get familiar with it first.