Add Get interface for sst_file_reader

[JingYanchao]

When integrating RocksDB, I wanted to use only the SSTable data structure. During this process, I had a need for lookups and wanted to use a Bloom filter for filtering. The table_reader already provides a Get interface, but it's not exposed as a public method in sst_file_reader. I've created an interface to expose this functionality.

[meta-cla]

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