Summary:
Add `SstFileReader::Get` (single-key) and `SstFileReader::MultiGet` (PinnableSlice) overloads to enable zero-copy point lookups directly from SST files. The existing `MultiGet(std::string*)` is refactored to delegate to the new `MultiGet(PinnableSlice*)`, which writes results directly into caller-provided `PinnableSlice` values instead of copying through an intermediate buffer. The single-key `Get` uses `TableReader::Get` with a `GetContext` for efficient single-key lookups without the overhead of MultiGet's sorting and batching machinery.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/14500
Test Plan: - New unit tests
Reviewed By: xingbowang
Differential Revision: D97825648
Pulled By: joshkang97
fbshipit-source-id: 17f3edd59bbf4747d17309c44ef12f0d952ea4eb
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