opt: improve cost model for lookup joins

[cucaroach]

The lookup join cost model mostly falls back to the index lookup cost. As the joinreader gains new functionality (see #51576) we want to update the model based on some empirical data from some well known workload. As a corollary to this work it would be good to look at adding table statistics to the regional_by_row tests that are relying on lookupExpr based joins. The issue at hand is that the cost model doesn't distinguish between a partial index and a full index w/ lookupExpr. Intuitively the partial index should have a lower cost based on having fewer columns and reduced I/O costs but in practice they come out roughly the same.

Jira issue: CRDB-8561

[github-actions]

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