Summary:
Use the user key on sst file for blob verification for `Get` and `MultiGet` instead of the user key passed from caller.

Add tests for `Get` and `MultiGet` operations when user defined timestamp feature is enabled in a BlobDB.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11105

Test Plan:
make V=1 db_blob_basic_test
./db_blob_basic_test --gtest_filter="DBBlobTestWithTimestamp.*"

Reviewed By: ltamasi

Differential Revision: D42716487

Pulled By: jowlyzhang

fbshipit-source-id: 5987ecbb7e56ddf46d2467a3649369390789506a