sql,spanconfig: temp indexes for index backfill should share a configuration with the final index

[ajwerner]

Is your feature request related to a problem? Please describe.

As described in #70426 / RFC and implemented in #73878, we'll soon create a new "temporary" index which will be written to during index backfills. Users can configure zone configurations for indexes prior to backfilling them (this can be useful). Any zone configurations applied to an index should also be applied to its temp index.

Describe the solution you'd like

We don't create these temp indexes until after span configs are in use. In principle, it shouldn't be too hard to pair a temp index with the backfilling index (I think we need a better API, but it shouldn't be hard). I'd like the sqltranslator to apply the same config to the temp index that it applies to its corresponding backfilling index. This should be easy.

Jira issue: CRDB-13192

[blathers-crl]

cc @cockroachdb/bulk-io