What would you like to happen?

As of now, BeamSqlSeekableTable has 3 methods, setUp, seekRow and tearDown,
seekRow is the most important method to BeamSqlSeekableTable in JoinAsLookup.
However BeamSqlSeekableTable doesn't know any details of the join condition before it startup. In many cases, we'd better to know the join keys for a lookup table.
With join keys, we can build a local cache based on them when startup. So I recommend that we push the join condition to BeamSqlSeekableTable

Issue Priority

Priority: 2 (default / most feature requests should be filed as P2)

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