codeintel: Expose index logContents

Created by: efritz

This helps with indexer/executor observability. The executor is sending back log contents (free-form text during execution) that is stored in the lsif_index record. This value is not currently viewable (without direct database access) and is hampering the code intel team's ability to easily troubleshoot the auto-indexer.

Our plan is to add this log content into the UI, but we will do so only for authenticated site-admins (all other users will see empty log contents). See the security notice here.

cc @sourcegraph/security I'm applying special care here because the current log text is not sufficiently sanitized, and ensuring that the text is safe will be an upcoming task on my plate (let's chat about techniques soon). Until it is, this will only exist in cloud and only be available to site-admins in that environment.