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

It'd be valuable for testing and debugging to have a primitive exposed to SQL to scan an index and return some representation of the KVs it contains. Today we have syntax to try to force a scan on an index but it doesn't provide low-level access to the KVs in the index.

Describe the solution you'd like

I'm not exactly sure what sort of syntax would be best. We could have some new syntax in the grammar or we could have some sort of magical function.

Jira issue: CRDB-3275